Most of us enjoyed the older DOS games of Doom and Heretic, but they don't work well with modern setups. Take heart! Rejoice!
There is a program known as Zdoom that works very well with Windows 95, 98, 2000 and XP! Works well with almost all *.wad files, only a few exceptions. A user-configured menu can enhance the default 640x480 video up to 8x better. Sound also is compatible to whatever IRQ is now used. Get a DL of Zdoom from this site, or other known-good sites.. http://zdoom.org/Download
And read up on this WIKI link about it! Yes, even available for Mac OS X users!!
Lots of modded *.wad files available for DL, for Doom 1, Doom2 and Heretic.
But a caution.. the original game programs are copyright, and even if past their "expected" life use, they are still the legal property of the developers! Stay legal, don't pirate programs.