I have had the same problem for so long I gave up trying to fix it and bought another computer which recently had the blue screen of death, but eventually I fixed that, took a couple of days to suss it and then last night I felt inspired so decided to have a go at fixing the old advent XP too. Today I tried all the usual things safe mode etc but there just didn't seem to be anything there and the computer still wouldn't start, so I tried all the F nos as it booted up to see what they were hiding. F11 had a change the boot order screen and I have the original recovery discs, but that didn't help, F12 had something else but that didn't do anything, F8 had all the safe mode and recovery bits but they were no good I had already tried all the different options before, but then F10... that was hiding something called PC Angel which could restore my computer in 3 different ways, one which kept everything as is but working, one which stored the current system and restored factory settings without formatting and another which formatted the hard drive so obviously all previous stuff would be lost. The first option wouldn't work because of a windows service pack,update and now I presume that that is the update that killed the system in the first place, but the second one did so now I have all my files backed up in one folder and what is otherwise effectively a new computer. So now the computer is working properly for the first time since October. It's worth checking if your system is hiding something similar that might help, I was on the verge of throwing mine away.