You need to think back on what you recently added or changed on the PC ??? did you update drivers ? seems like lynchknot always gets a visit from Dr Watson when he updates his ATI drivers. Don't you get some kind of error log that suggest whats causing the problem ?

System restore doesn't work ? XP Repair will fix the OS but not driver/software related things, but its worth a try incase the OS is corrupt in its interaction with such.
it's getting really anoying and i cant think of any way to fix this besides a format which is impossible.I have to much important stuff on my PC to risk it.
One partition ? no backup or image ?