lennyx :
did that, where would the error appear, the pc still just restarts, thanks
hmm, the error should appear on a bluescreen instead of getting a restart.
You might try looking at event viewer. Just type in 'event viewer' in the search pane that comes up when you hit the start button. Look at right after a crash and see if there are any recent errors listed.
If that does not help you may have to start checking hardware in a process of elimination and see if you can narrow it down. Running 'memtest86' would rule out the OS as a possible cause. See if memtest will run a long time with no errors.
I'd try undoing any recent system changes or software or driver installs.
If you can't discover any hardware problems such as faulty start or reset switch on case, overheating CPU, flaky power supply, bad RAM stick, bad hard drive or system short etc. then maybe try 1. system restore (control panel > backup) 2. a repair install of Vista and 3. finally a reformat and reinstall.
If the final option doesn't work then it's back to hardware checking. Sorry to be so general, but you a problem with many possible causes.