Your hard drive will be being used nearly all the time you are doing something. Browsing the net will be putting cookies and temp files on there, and things like AVG will be running in the background and may write/read files.
Powering down the hard drive is (IMHO) always a good idea. It will mean there is more chance of it failing and its often when coming back up that you have problems. Things may be better in XP but under previous Windows incarnations this options has often caused the PC to freeze forcing a cold reboot.
Perosnally I don't think you will gain much.