my computer freezes after about an hour or sometimes earlier after bootup.. im not sure but i think it is my ram. ive got 2 rams…256 n 512mb ddr ram..when i boot up it says approx 490mb ram..this is cz 32mb is for onboard graphics..so i thk my 256mb ram is at faulty..however whn i use only the 512mb ram it still freezes after an hour or so…my pc screen jst freezes…the mouse / keyboard dont respond at all…
Run Memtest86+ - Advanced Memory Diagnostic Tool to check the RAM, put one RAM stick at a time to figure out which one is faulty 
i ran memtest, but it froze in tat too, it showed 2 passes but it ddnt show any errors…it was in its 3rd test but it never made it…wat cud b the prob??
problem solved…i installed a new 512mb ddr ram together with the old 512mb ram…n now my pc doesnt freeze…it ran whole day without being freezed..