Do NOT reformat your PC. That is very unlikely the cause, and will be a waste of a lot of time and effort. It never hurts to run a virus scan, but I highly doubt this has anything to do with a virus, and unless your PC is turning off at any other time, you power supply is totally fine.
Firstly, you did not mention any details about the PC you have. I am going to take a guess that this is an upgrade or new installation of probably Vista on a built PC - not a Dell, Gateway, etc., or possibly XP. If it is Vista, Vista has major problems resuming from sleep modes. This is all over the web. Installing service pack 1 for Vista will likely fix this, if it does not, your BIOS may not be configured properly for Vista or even XP. Go into the BIOS, usually you hit the delete key as soon as you PC boots, and make sure the option for "ACPI" is set to "yes" or "on", next, take a look at you "S" setting. This should be under power or system, something to that effect. You choices will be S1, S2, or S3 - these are the different standby states your motherboard supports. If you are using XP, or esp. Vista, make sure you set this to S3. This will likely fix the problem.
If you have had this installation of Windows for a while, and this is just in the last week, I would use System Restore and go back a week or so. You will not lose any data, but if you installed any programs since whatever date you select for the Restore, you will have to reinstall them. Depending on your tech level, if you have XP, you can do a "repair" installation, if absolutely necessary. This will preserve all your data and programs, but reinstall all the system files. You will have to install about 9000 updates after doing this, however. For directions of how to use System Restore or how to do a Repair Install, just go to http://computing.net and search on those exact phrases for your specific version of Windows.
Good luck!