There is a function supported by most PC-s called suspend-to-ram. This means that when activated, the whole PC will shut down (cpu fans, hdd-s, psu fans, etc) except the ram, that will keep a record of all opened applications. When you power up again, the PC will start really fast, and you will be in windows directly, with your applications opened. This is improving the default Windows Suspend mode (normally it only shuts down monitor & hdd-s). This is the recommended shutdown option for someone that wants a quick break & resume.
Depending on the motherboard and the BIOS (my old nforce2 with award BIOS has it), you can enable this simply by entering motherboard's BIOS configuration, and somewhere at Advanced Power & Configuration to find a setting named Suspend type wich defaults to S1. If you select there S3, then when you select Suspend in Windows, it will do the suspend-to-ram thing. The PC will be totally quiet, only the ram will be powered.
And now comes the easy part, in Windows, at Display Properties, Click Power button near the Energy picture, and at Power Schemes, select System standby after...your 3 hours. All done!
Ofcourse there are software programs than can schedulle a normal system shutdown, or the solution offered by Riser that envolves some work, but those cant beat the fast resume in windows that suspend-to-ram can offer. Too bad not many use this.