sometimes you can take the program files folder that holds the game you want and copy it to an external drive, then after initial install of the program on your new OS disk, copy your original data to the new drive...
all the old data overwrites the new data, and you have your flight simulator working just as it ever did. this doesn't always work perfectly, but it is worth a shot.
as far as xp, you should be able to activate your new setup with a simple call-in to M$ if you can't activate the regular way.
the time you spend doing a clean install is worth it.
my main rig...
asrock 970 extreme3
AMD athlonII X3 440
zalman cpns5x performa hs/fan
crucial ballistix 2x4gb sport ddr3-1333
powercolor ax7750 1GBK3-H vga
antec neo he 650r
Samsung 840 EVo SSD 120 GB
toshiba 2TB HDD 64M cache sata3
seagate 1TB HDD 64M cache sata3
hitachi 2TB HDD 64M cache sata3
lg wh14ns40 bd burner
optiarc ad-7240s sata dvdrw (nec chipset)