forgot to say that the CPU is a Celeron 2.93
XP Home SP3
250GB HDD (new)
Stealth ATI Radeon PCI (new)
450W PSU (new)
Intel MOBO D845GVSR
At first it seemed the old graphics card was the problem, but then it went back to old habits, so I replaced the HDD because I noticed the HDD indicator light was not flashing at all - that didn't make any difference; same with the PSU.
I've updated the BIOS, Graphics drivers, ran the Intel trouble shooting wizard, and this is driving me crazy! It still doesn't like to reboot after it's been running for more than an hour. Sometimes hitting the 'reset' button will do it, sometimes not.
Any other ideas?