AVG isn't worth a wank. Get it uninstalled and load Avast.

Sounds like you've got some process or other loading on Windows startup thats hammering the shit out of your machine. More than likely a trojan of some sort.

Run 'MSConfig' from your run command (just type it in and hit OK). Look at the services and startup tabs. Make a note of any that arent Microsoft or clearly identifiable (such as printer/camera utils and the like) and google them. You may find a virus/trojan amongst them, in which case google should also point you at a way to get rid of it.