Hugh;
You gave insufficient information, but I will make a guess at a common
cause for that problem.
Otherwise post back with details.
Does it fit this: http://support.microsoft.com/?kbid=326904
If it does, you most likely have a pirated Windows XP Pro.
If it is pirated, you need to contact the seller and/or Microsoft: http://www.microsoft.com/piracy/reporting/default.asp
One of the features of Service Pack 1 is to detect pirated copies of
windows XP.
Also you have 3 options to become legal:
1. Go back to the previous OS: http://support.microsoft.com/?kbid=303661 http://support.microsoft.com/?kbid=314052
This may or may not work depending on the installation options used.
2. Perform a Clean Install with the OS of your choice destroying ALL
data.
3. Purchase Windows XP Pro (Home will not work) and perform a Repair
Installation using the new valid key and CD: http://www.dougknox.com/xp/tips/xp_repair_install.htm