semi predictable sudden reboots

[frzndmn]

i have a pc that's having sudden rebooting problems though the symptoms suggest (as in for as far as I know) software problems, e.g.

always reboot after symantech finish scanning a drive partition
always reboot when scannng using spybot
it had this problem before and it was "fixed" for a few month using a system-restore-like program
after it reboots when viewing a certain webpage(pretty random, I visited pages in same site safely using other computers), it will always reboot when I come back to it after pc restart, and usually dont for sometime if i don't goto that webpage.
if I dont actually do anything using it, it wont reboot even if I leave it on for a day or so

so would a normal hardware problem case have these symptoms? or is it definitely software? or is there not enough information?

thanks in advance