Hard Disk problem

Dear sir,
I am facing the hard disk problem? When start the computer, booting the system & show the Windows XP logo then further automatically reboot the machine and its 2-4 times reboot after that enter the Desktop.

I thing its hard disk problem that why I do the Disk Defragmenter the "C" drive. When I use this system then give the message "Could not defragment this label" but when I Disk Defragmenter the another drive that time does not give the any message, then work properly. Why it show the message particularly C Drive & what is the exact problem in my system and how could I ignore this problem. If you solve this problem then I really regards to you.

Thanks.

Subhajit