You're off topic for this forum, which is dedicated to Windows Live OneCare.
I'm confused about why you would be asked to activate XP if you installed Windows 2000. The product key for Windows will be with the CD you installed the operating system from. However, if you are being prompted to activate Windows XP, you didn't install Windows 2000. I suggest contacting the computer manufacturer for help with how to restore the operating system on the box.
-steve