There are many causes to the "unknown software exception 0x0000005".Some are hardware base and a few software based.
Its a case of hit and miss, narrowing down the field ussally helps find the problem.
Few ideas:
Try running MemTest. Either burn to a CD or write to a floppy and boot from it. Let it run for a few hours (minimum 5 passes is my recommendation), so probably overnight unless it finds errors sooner. If it doesn't, then it could be a heat related issue (CPU overheating). Might check your HSF and see if it's seated properly or needs more thermal grease. Could also be a faulty PSU. Bad power can cause all sorts of issues. CPU is the next thing to consider (bad processor leads to bad output). As you can see, there's a lot of things that can cause a crash.
If running XP sp2 check for any new software that runs DEP
Data Execution Prevention and turn off the DEP option.