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Check the time it takes to go into standby about 3-5 times. If it's the exact same time, every time, then you've probably (or some software has) set the pc to go into standby automatically.

If the time it takes is irregular and it's going into standby whenever it feels like it then open up your case and check to make sure all the fans are turning (cpu, psu, gfx card & chipset fan if applicable) as it could be an over heating problem, though this usually shuts the pc down rather than putting it in standby.

Also make sure you're virus checker is fully upto date or try scanning your pc with a different virus checker from the one you're using now as some are better than others. Try You can scan your pc online and it won't install anything perminently on your hard drive. Sepending on the size of your file system it can take a long time so be patient and wait until it tells you it's complete. It will also give you instructions on how to remove a virus if it finds one.

Before any of this (if you have XP)

right click descktop>properties>screensaver>power...>power schemes> and set the power options to whatever you want. They're self explanitory.

Let us know how you get on..........

Well my first thought is Sasser or Blast viruses (virii?) so I would suggest a third party scan:

Norton
Housecall
Panda

Of course you need to stop your PC from shutting down first:

Start Menu > Run > shutdown -a

If no virus is found it could be other problems

Oops wrong Norton link - use the one provided by Sam Slater. Though I recommend Panda (unless you are using Firefox)

Hard luck, mate. 60,000 + Windows viruses - what a fucking nightmare.

Well then it sounds like your firewall is set wrong. Disable it just to do the scan.