Crashes = blue screen?
If yes, do you have any (recent) files in WinNT\Minidump folder?
No blue screen ...
Normal events:
a) Turn PC on & BIOS completes.
b) Everything okay to the log in screen.
c) Select user account, enter password & click enter.
d) The screen goes black for a few seconds (as is usual during start-up) until it comes back on with the desktop.
However in this last case (d) something must happen during the screen being off because the next thing you see is the BIOS starting back up, i.e. the PC has decided to reboot.
The only way I can get it to work again is to go into safe mode, and use system restore. After that the PC works okay, but Avast Update kicks in, and Zone Alarm asks me to accept or deny it. If I accept then when the PC is next turned on I'm back at the above scenario.
I even thought of reinstalling Windows just in case but that feaks out with the 'this version of Windows is more up to date than the one on the CD' so it won't allow this.
This problem has only happened since I accepted that latest program (not virus) update, everything before then was working brilliantly.