I have a problem and have no idea what its cause may be.

Since Friday, I've had problems with my computer.  At first it seemed like something was wrong with my Internet connection, because things were going much slower than usual...and I'm on Telecom Broadband.


However, it was probably around Saturday night when the system started to freeze after so long.  It began to look more like a hardware problem.  I was playing a round of a rather large PC game (Civilization IV), which is already a bit sluggish due to its size, but it was much slower this time around.  It must've taken more than 10 minutes for the AI player to do what it does, which normally (on my system) takes close to 5 on a bad day.

I used to be able to freely browse (on Firefox) with multiple tabs open without a hitch.  Not so much now...

Last night I had a couple friends over, and we were looking at stuff online.  In the space of maybe 2 hours, I rebooted at least 3 times!

Can somebody tell me what the problem may be?  I've tried updating AVG, Ad Aware (which failed to update, by the way), Spybot S&D.  Things seemed fine at one point (between 2 & 4am...not good), but today it's back to being a pain in the jacksie.

Help!