CPU problems

I going to assume that by "cpu" you mean computer, not central processing unit.

I would like to help you, but I need more details before I can diagnose an issue. There could be a million and one things wrong with your computer at this point, what else is going on?
How long have you had the computer, what kind is it, did you just build it, what are the components inside?
details, details, details.

Sounds like a failing PSU, but more details are needed as mentioned.

Hi, When i said CPU, i meen the central processing unit only, when i switch on, some times it goes upto windows and hang by itself. My cpu is Intel core i3, i am using a intel mb and 2GB ram. I build it myself. Is this because of PSU? or the hotsink?

Not enough details. We need you to be exact and unambiguous.

If it is an i3, is it an i3-2120 or an i3-2100?

If it is 2 GBs of RAM, is it 1x 2GBs? or perhaps 2x 1GBs? or 4x 512 MBs? Is the speed 1333 mhz? 800 mhz? 1600 mhz? What company made it?

What is the exact part number of the motherboard? The case? The PSU?

We can't tell you if there is a basic incompatibility between your parts if it isn't crystal clear to us exactly what they are.

The only two things I could ask you to try at this point with so little information would be to 1) Push the RAM down harder and turn it on again, or 2) try with a better PSU.

Note that higher wattage doesn't necessarily constitute better. Higher quality components also matters. A higher wattage one would be better than nothing if you don't know the quality, though.