Comp Freezes every second...

I've had this really annoying problem of my computer beginning to freeze unexpectedly after using it for a short period of time. I don't have alotta PC knowledge unfortunately. There is also like a squeaking noise coming from it periodically, the light that comes on accompanied by the computer actively doing something also flashes when the squeaking noise happens.

The freezing issue is a lot more aggeravating as i play online games (such as Cod4 etc) and it makes the game unplayable after just a short period of play. I also get slowdown that ISN'T lag related, I'm sure of that.

Cant remember the name of my motherboard but the rest of my rig is: Pentium 4 Processor (Intel Inside), 2 gigs of RAM, around 200gb of memory and, in case this is useful, Radeon x800 PRO/GTO. I use the free AVG anti-virus program which i recently installed (as Norton 360 wasn't letting me do ANYTHING, even with reduced safety settings. The problem was there before the installation of AVG however. Any help would be great, thank you in advance.:4-dontkno