Why does computer shut down?

I have computer with windows xp it is 9 years old. When ever i run any kind of scan the computer shuts off. no matter what kind of scan i run it shuts off. By the number of files that are scaned it shuts off half way thru the scan I have tried shuting off system restore does not help when i run an adware it shows a lot of bugs but i can't get them out because it shuts off ad ware virus scan spy bot they all run half way thru and it shuts down any good solutions

Why does computer shut down?adware

Try re-installing Windows XP, might be some of your system files are corrupted because you keep on shutting it off with the normal Shutdown or Restart process.

Why does computer shut down?antivirus scan

Increase your RAM ...but before that consider selling it off and buying a good cheap PC.
stop looking at porn!
to me this sounds like a virus, i have seen me install programs to remove them and the virus would delete it as soon as i installed it other then a virus all i say it could be is your power supply is bad. you can check hard drive also by running

start-run- chkdsk as this looks for eros on drive and fixes some

hope this was some help
Try format the drive which contains the virus. If the virus in the same drive where the OS in. Then format that and install new OS.