Frequent freeze.

My computer freezes frequently and randomly. It might be in the windows or before entering the windows. The screen just hang over there, including the cursor. Nothing can be moved. If that is an audio output at that moment, the particular tone continues. For example, 'hooray' ---> 'ay-ay-ay-ay-ay'
I have to reboot everytime it freezes. And twice, it showed 'no drive detected'. Otherwise, it just goes into windows as usual.
Checked RAM usage, core temperature, virus. Even reformatted.
And I realize if I shut down the PC for a day or two, it freezes lesser.

Specs:
Intel(R) Core(TM)2 Quad CPU Q8200
RAM: 4GB
ASUS P5Q3
NVIDIA GeForce 9800 GTX/9800 GTX+
Vista Home Premium 32-bit

PS: I'm bad at PC stuff. You guys gotta help me out. Thanks in advance. And I'm not sure which category should I post to.