Computer keeps looping and rebooting

Hi folks,
The problem is that, when I turn on my computer, the Windows XP logo appears with the colored dots scroll, then eventually goes off. The CPU fan seems to sound noisy, then it reboots. Later on, it's back to the same spot and reboots all over again. When I turned on the safe mode, it did the same thing. When I used an XP boot disk, still, nothing happened. It goes on continuously.
Please help.
Thank you.
