Something is causing a BSOD. To stop the reboot goto control panel, open
system, advanced tab, startup recovery, settings. Uncheck Automaticlly
restart. That
will stop the computer constantly rebooting but you will end up with a blue
screen. The blue screen will have error codes on it you need to write down.
It might also have a file name on it. Write down all the info and repost
with the exact error message. Someone will then help you diagnose the
problem
Looks like you did something wrong along the way, either in your preparation for the upgrade or the execution of the upgrade. Since you haven't provided any details that might help explain what went wrong, I would suggest you read the following for information about upgrading from Windows 98 to Windows XP: http://www.google.com/search?q=upgrade+from+windows+98+to+windows+xp
