When I stat computer & as windows XP logo comes system will suddenly restart... can anybody help me to solve this problem without reinstalling XP??????

Start your windows XP in safe mode.. To Start window in safe mode..

To get into the Windows 2000 / XP Safe mode, as the computer is booting press and hold your "F8 Key" which should bring up the "Windows Advanced Options Menu" as shown below. Use your arrow keys to move to "Safe Mode" and press your Enter key.

Once u start your compute in safe mode right click on Desktop and click on properties..Go to Settings and Change Screen Resolution to some appropriate value... click on advance then monitor...change it to 60 Hhz. This might solve the issue...

I was getting the same problem few days back.
Another thing which I noticed was whenever a disk check is going on, computer gets restarted as soon as it says "Press any key before 6 seconds".
To avoid this, I tried cancelling disk check before 6 seconds, but then again computer got restarted.
I encountered this problem even after running in safe mode.
Is it due to some virus???

Rajji..Is ur Hard Disk is in good condition.. How many Bad Sector it has?

No Idea!!!!!!
Got it replaced.
It was due to the hard disk problem.
Thx