CPU processor running at 100%

My pc runs extremely slow, my processor makes unusual noises especially when I put pictures on the screen animation (games, icons msn, etc. ...), I play a ground game (worth redundancy) online, it played perfectly and from 2 weeks ago I presented a tremendous lag, writing in the game chatting to incredible speed of 10 letters per minute .

I opened the task manager and I see that the CPU is running at 100% when I opened only a game or msn.

I have the last scan of NOD32 and does not detect anything, defragmented disk and stays the same.

my PC:

OS Windows XP Professional SP2
512 MB RAM
Intel Celeron 2.40 GHz

I hope response and a thousand thanks from now.

You will have to open your computer and locate from where the noise if the hard drive or a fan who was obstructed. If your graphics card is a PCI or AGP slot for checks if heated too much or if the fan is wrong (to take clear fan) .

Thanks for your prompt response.

I have already indicated and what the noise persists, opened the computer and disconnecting things until I was left with only hard disk and the motherboard connected. The noise is not in hard disk that remain uncovered what I put a lot of attention and the sound does not come from there, the sound is exactly the motherboard processor (I think) also disconnect the fans and nothing remains in the same. I do not have this graphics card is integrated into the motherboard that only the audio card and modem and the ram. Before I had formatted the system but now I have a lot of valuable information and is the last thing to be done.

The problem remains the same. that more could be?