Don't keep the tests to LFS only.
try cpu benchmarks, ram benchmarks, graphics benchmarks.
Without loosing much time, using the current windows configuration, try:
superpi
everest
3dmark (try 2001 version, its dx8)
Compare the results online to see if you are close. Try to compare to the same cpu/graphics model running at the same MHZ (most of those guys are overclocked).
Write those results down on paper. They are your new standards to compare when you do the following:
Remove hardware from the system, or disable it on the motherboard Bios. Disable sound, run the benchmarks. Disable USB ports, run the benchmarks. Remove every peripheral except keyboard/mouse, run the benchmarks.
If this fails, format and try winxp with different directx (don't update to the last dx9).
These are basic diagnostic procedures you should do.
Use hiren boot cd, to ghost your c: so you don't loose time installing everything again.