XP Shutting down on it's own.

Possibly a virus. It should be patched before surfing the web.
Another thought is a BSOD (Blue Screen). If your system is set to auto reboot, this could be the cause. If it is a BSOD, then there is probably a driver that needs to be updated because XP doesnt like it very much.

Is it shuting down when you go online-if this that you have sasser (virus)

Or go to Start and clcik Run type cmd
than enter shutdown.exe -a this is to abort the shutdown.

Sorry, all I have is more questions, but answering them should lead you closer to a solution.

Shut down or crash?
how often?
During activity or inactivity?
Anything in event logs?
Is it perhaps hibernating (power management)?
Is it overheating? (bad fans)
Is it connected to a UPS (failing battery or scheduled shutdown)
Does it do it in Safe Mode as well?

Check it with anti-ad, anti-spy, anti-virus programs.
Use HijackThis to determine what's being loaded at startup.
Check the task list and investigate any odd tasks (refer to a working system and the internet to determine what "odd" is)

If there are problems with hardware or XP anywhere, your system may be set to reboot when it encounters a problem.

Go to Control Panel, System, Advanced Tab, Startup and Recover, Settings, and make sure the X is OUT of the Auto restart option.

That way if your system has a problem, it won't auto-reboot; it may freeze or give you an error message instead.

Good luck.

When you say that it is shutting down do you mean that it reboots or it actually shuts off.

If it actually shuts off, it could be a hardware issue, possible the power supply.

If it reboots, then it is more likey that it is a software problem. Quite often this type of problem is caused by bad, or wrong drivers, esp. video drivers.

More information about the shutdown would be helpful. Eg: when does it happen, how long has the computer been turned on, what were you doing on the computer when it shutdown, how often does it happen, etc.?

first thing plz tell me what kind of problem with in detail like RPC error and shutdoing or at a timin of booting or at a time of logon may an hardware device driver cause due reintallation of xp and overwriting to old files...

plz verify and run safe mode and try to reinstall that device driver... after that in normal more install again...

also keep in kind to save system files whiles service pack & related update bcz u need now to roll back service pack and related and then reinstall of them

check the capacitors next to the CPU.
If the capacitors are flat on top they are good.
If they are raised they are bad.
Having just one bad capacitor will cause this problem.
You will have to replace the motherboard

Good Luck