Re: Desktop restarting every 5 min
Could be any number of things, CPU temperature is a possible candidate but it's definitely not the be all and end all of the options.
Easy one to confirm or deny though, shouldn't need to download speedfan just to look at temps.
As your machine reboots, head into the BIOS and look at the temps there (may be in somewhere like hardware information, just have a look around.) If they're very hot, open up the case and check the fans are spinning as they should. If they're not because they're clogged up with dust or something, get rid of it and see if that helps.
If it's not a temperature issue, I'd also recommend running a full virus scan as a matter of course.