Why does my computer freeze?

I am using dell CPU with Intel dual core. I use windows windows XP service pack three and have original nod32 antivirus. So here is the problem , when i tart my PC at any time it completely freezes. the mouse doesn't work. The keyboard doesn't work. My internet modem doesn't detect any PC activity. Nothing works. And this freezes can happen at anytime: when the windows XP loading screen comes, when the pword screen comes, when I'm playing games, when I'm surfing the web (tried different browsers) or when I'm listening to music. I tried 3 different antiviruses but they don't detect anything. I used tune up utilities but still nothing. Please tell me whats wrong with my PC. Also i don't know if this will help but a click sound comes from my CPU, like its reading a disk or a program, just before it hangs. Please help