The Mother Board

My Asus P5p800 se mb that worked about decade seems to be fully unrecoverable. The used p5p800 se mb I can buy far from my home, even far from my country. But I plan to substitute it by similar one, used already, as I would not like make full PC Upgrade. So I want to ask what fine substitution could to be? For example -- P5P800, p5-p800-mx -- do they have any limitations.
What another options could you propose?
The characteristics of p5p800 se relating to my PC other componentss are -- Intel pentium processor, i865 chipset, 775 socket, ddr1, Afp videocard, IDE hdd.

Certain that the board cannot be revived? Common fault is the BIOS battery going dead, and system settings are lost in CMOS memory. Try a new battery, reset configuration.
Also look over all capacitors on the motherboard. Any that have domed tops, lean excessively or show leakage are bad.
That Asus P5p800 se may not be affected, but do check those caps nearest the CPU, as excess heat may have affected them.
More info about "bad caps" at www.badcaps.net/
As to replacing with a newer board, you likely will need to reinstall the OS, unless you find a model with identical chipset types and features.

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Here I already asked about buying a new one. I do understand about the same chipset, socket, hdd type to avoid the OS reinstall, despite it is not so critical. The Agp socket to use the same vcard, what else I need the same?
I found in my area the P5p800 without workable LAN -- is it possible to recover its feature? Could I not use this feature as I do not have any LAN connection, just 3g usb-modem for mobile internet?
Talking about the burned mb -- it was due to serious surge of overvoltage from burned psu. The hdd was recovered just by unsoldering TVS diod -- Can I find same way out on the mb? Can VRM module was just burned?
I have already passed to service repair - they replaced bad transistors or capacitors -- but no beep, no post, just fan spinning. Could the CMOS reset by jumper (indeed it has normal voltage) and bios replacement help in this situation, assuming that voltage surge burned them. Meanwhile it is improbably as a lot of other components in the circuits should be toasted as well. So the reason can be in the input electric supply area on mb.

If as you note the power supply (PSU) went bad and damaged the motherboard, then little hope of reviving it. The CPU itself may have sustained damage. Only way to know is to test it in another motherboard that supports it. Some CPU circuits work on only 1.5 to 3.3 volts.
Most circuits work on only +5 volts, if that goes up by more than 2 volts due to bad PSU, then the IC chips may be fried.
Chipsets and features that are similar or identical on another board may get you back in operation.
As to using USB for Internet, should be easy if you use a WiFi wireless router or hub. Or an add-in PCI card for LAN.
You might be lucky and find an exact replacement on eBay or other source for a decent price. Ensure the seller warrants the board is good!

_________________
F@H.. to solve mankind's maladies.. in our lifetimes!

It would be fine to see functional/structural diagram of my main board pcb, at least the chips and other components around electric supply and socket on mb. Should processor fan is spinning just mean that it has electric supply and it shows nothing about the cpu state after psu change? I did not know did mb burned - the service replaced some visually ruined component and electric supply on mb- so probably vrm shooud be fine. But no more. Then - on hdd just one tvs diode wast broken after this surge. I am wonder if I reset cmos and bios chip reflash - should it means I need to reinstall OS anyway?

Each IC chip has three lines of info.. brand, model number, and date of creation. Normal color of that info should be silver-like. If dark or brownish in color, may have sustained heat damage.
Normally, if the CPU fan is temperature-controlled, it may spin normally for a few seconds on start up, then stop or slow down. If it spins normally all the time, then it is in a manual setting.
If the original PSU failed, then try with a known-good, quality PSU to see if there is still life in the board. May only be a one in three chance that is so. Some system brands were notorious about when a PSU failed, the motherboard and other features were rendered useless.
As to a "tvs" diode on a hard drive, that would be for reverse-voltage protection. If failed, the hard drive controller board may be useless. You can replace the controller board with one from another identical hard drive, to recover data.
If all data on the hard drive is secure and not corrupted by the incident, may be possible to get the OS back in use with few problems, maybe just a system check.

_________________
F@H.. to solve mankind's maladies.. in our lifetimes!

I have connected to known-workable psu after that. And my hd works after "deleting" broken diod. In this vein I would like to know can this overvoltage ruin the cpu (taking into account 12v direct supply ) as well as I did not find relevant mb to check despite I was said that pentium does not burns. Can bios reflash and cmos reset to help? What do "system check" mean?

As i already said I found just P5P800 board to buy but withoul workable LAN(rg-45) port (it also has one more pci socket and larger board complarably to my SE edition). Should I install original driver for this mb, could it block my other pc components when I connect to this mb at first time due to nonworking lan - due to absence of some kind of system check?

"System check" refers to a process on the OS install disk, known as SFC.. or System File Check. It compares installed files with original, and if missing or altered beyond use, replaces them.
https://support.microsoft.com/en-us/kb/929833
It is best to use the motherboard driver set for the specific model at hand. While most features may be identical from micro to full size boards, there will be some differences. Some drivers may not work properly if from a different board.
Components such as transistors within a CPU design may be intended for only 1.5 volts power, and if much higher, can fry them. If in doubt about a CPU being healthy, test in a known-working system that it is compatible with.
Newer CPU types since the Socket 775 have the memory controller as part of the CPU die.. not a separate North Bridge chipset feature. Memory modules must be rated at lower voltage than with previous CPU types.
Flashing a BIOS should be done only if the original is corrupt, or there are newer compatibility standards for use. The exact version is mandated, or there could be a fatal failure of a working system.
Normally, just a reset of the BIOS and CMOS memory will restore life to a system. Basically, no AC power input to the PSU, drain it of reserves by attempting a few starts, then remove the BIOS battery and use the provided jumper switch for a few seconds.

_________________
F@H.. to solve mankind's maladies.. in our lifetimes!

Could the mb bios chip to be corrupted due to overvoltage from psu? I have read today on one forum that after replacement of tantal capacitor on mb it needed the bios reflash to launch as a result of new attaching new components- maybe it is in my case as service master replaced several capacitors, drivers or transistors. 2. If I would take another P5P800 (non SE) should I just do the OS reinstall, or I need also bios reflash by floppy disk for new pc configuration? 3. I also tried to check my Pentium 2.8 hz some days ago but master was unable to find appropriate mb (or cpu may really have defect). In this vein I would like to ask - should my pentium to be tested with i865 chipset mb as it worked with the same one on my mb, or there is no such correlation between type of cpu and chipset?