System restarts automatically

My system without restarting, restarts automatically.i doesnt restart in usually way. It restarts as if power is switched off and on again. Can any one give me some tips.
My system config
Windows XP,
Processor,
800 megahertz Intel Pentium III
32 kilobyte primary memory cache
256 kilobyte secondary memory cache
Main Circuit board,
Board: i810-W83627HF
Bus Clock: 133 megahertz
BIOS: Award Software International, Inc. 6.00 PG 05/02/2001
Memory Modules
256 Megabytes Installed Memory

Slot 'A0' has 128 MB
Slot 'A1' has 128 MB
Slot 'A2' has 128 MB
Slot 'A3' has 128 MB
Controllers,
Standard floppy disk controller
Intel(r) 82801AA Bus Master IDE Controller
Primary IDE Channel [Controller]
Secondary IDE Channel [Controller]
Bus Adapter
Intel(r) 82801AA USB Universal Host Controller
Multimedia
Intel(r) 82801AA AC'97 Audio Controller
MPU-401 Compatible MIDI Device
Standard Game Port
Desktop system type.
Can any one give me tips so as to how to protect my computer from viruses. I am at present using norton anti virus.
More over my system is operating very slowly.Before it used to operate quickly.We brought it 4 years back.Now it became sluggish type.can any one give me cure for this thing?
Thank you
Naveen

 

This is how to protect yourself from malware. Very good thread to follow.

But, it should be followed by a visit to this thread, to make sure you're free from malware.

By following these two threads, you'll almost assuredly be free from malware (ie viruses, trojans, spyware, etc). Then you can examine your hardware, if the problem remains.

The first step should be the easiest. I would recommend that you replace the CMOS battery. They only have a rated lifespan of about 3-5 years. Its a simple fix, and should only cost about $5.

Then, I would recommend that you check your power supply. Ensure that it is providing the correct voltages. Finally, ensure the system is clean and that no components are overheating, which really shouldn't be a problem.

If you have any more questions, don't hesitate to ask.