Operating System Failed To Load Normally

PC is left on pretty much all the time. Whenever I have to restart it for whatever reason it restarts normally, loading XP relatively quickly. However, if I have to shut it down for even a few minutes, Windows will typically fail to load most of the time. Usually it will hang on the black screen with the white progress bar, or sometimes on the loading xp screen. At this point it will sometimes start up when I reset it, or it will show the "Windows failed to load normally" screen. If I then start in safemode then restart it usually works, or even if I choose "start windows normally". I'm not sure if this is a driver or hardware/bios issue, I have reformatted once, and restored the OS using my XP disk a couple of times but the problem persists. I keep most of my drivers up to date. Tools like Sandra don't report any major errors (from what I understand anyway). I have just ordered new RAM, as I thought this would sort the issue, but it hasnt. I have to run my memory at 100hz so it will boot without any problems . If I run my processor at 200fsb then the computer doesn't boot and it doesn't start.