If you think that your Windows XP copy is counterfeit, go to the following address
http://www.microsoft.com/en-us/howtotell/cfr/Report.aspx
Then buy a Retail license of Windows XP (or any other Windows you want) and perform a new full installation.
If you think, instead, that you're having a Windows XP validation issue, post your question in the Windows XP Genuine Advantage Validation Issues Forum at the following address
http://social.microsoft.com/Forums/en-US/genuinewindowsxp/threads
providing detailed informations about your system and the problem you're experimenting.
Bye.
Luigi Bruno - Microsoft Community Contributor 2011 Award