Please give us the exact error message and precisely when you are receiving
this message. Does the message appear during setup or during activation?
If it's during activation, instead of using the Internet option, use the
phone option. When you call, explain what you have done and they should
supply you with a new activation code.
Also, if your system came with XP pre-installed, be sure to use the key
located on the sticker affixed to your CPU case.
If the above fails, please check the following Knowledge Base Article to see
if it applies to your situation:
http://support.microsoft.com/?kbid=326904