When the computer boots press the F8 key and see if you can disable the
Automatically Reboot on Failure option.  Disabling the Automatically
Reboot option may cause the computer to halt on a Blue Screen bugcheck
error screen.  If it does copy and post the information from the error
message to your next post.

If the computer does not halt on a blue screen error please give us more
details of what happens when the computer tries to boot.  Does it make
it as far as the logon screen?  Does it appear to load the desktop then
reboot?  Your computer may have been infected by a virus but we need
more information to provide further help.  Also, do you have a Windows
XP CD?

John