Most times, it does this a few times in varying intervals ranging from 1-5 seconds before ultimately freezing all together. I don't get a blue screen or any other type of message. I've also noticed that it only seems to happen at most once per day, and that it's more prone to happen if I'm playing a game, although it has happened while watching movies and also when the computer is completely idle.
I've checked the harddrives, and I've also run memory diagnostics. It doesn't seem to be directly temperature related, seeing as it can happen when I'm not even using the system.
Windows Vista x64 Ultimate
MB: Asus m4a78-e
CPU: Phenom II X3 BE
GFX: Radeon HD4870
Memory: Corsair XMS2 Xtreme TwinX DDR2 PC6400/800MHz CL5 2x2GB
HDD: Samsung Spinpoint F1 (300GB + 750GB)
Any help is very much appreciated!