hi, welcome to DAL, first off, we will need a bit more info,
do you get any error messages before it shuts down?
have you scanned for viruses with an anti-virus program? (you can use your own or download AVG antivirus by clicking the link in my signature labled AVG Anti Virus Free Edition)
is this a new problem? if it is you could try system restore to restore your computers settings to an earlier time (info here:
http://www.microsoft.com/windowsxp/u...emrestore.mspx
This could also be a hardware problem (e.g something overheating)