Computer screen freezes, blacks out and then returns back to normal

Basically, when i'm surfing the internet or playing a game. My computer screen freezes which then results in me 'Ctrl+Alt+Delete'ing which then makes the screen black out and then brings it back to its normal state.
I was wondering if its due to an overload in my Hard-drive(90% full) or the fact i have a low amount of RAM. Any advice given into what to do is very much appreciated. My spec is below;

Intel Pentium 4
Motherboard - ASUSTeK P4S800
RAM - 512mb
Graphics Card - Geforce 6800XT 256mb
80gb Hard-drive

Clean the hard drive of important data and only run what you need. An 80GB drive should be plenty if you watch what you are storing on it. Videos and games take up a lot of hard drive space. I run 2 drives. A 400GB and a 250GB. Bump your memory to 1GB