Try this
1 . Go to Start > Run > Type “Regedit” (without the quotes)
2 . Go to the key:
HKEY_LOCAL_MACHINE\SOFTWARE\MICROSOFT\WindowsNT\CurrentVersion\WPAEvents\OOBETimer
3.Double click on it and change the value to anything but the value which is already recorded.In some cases you won’t be having OOBETimer ,so follow the given step if you come under that case:-
4.Right click on the right side of “WGAEvents” and select New > Binary Valueb.Rename it to “OOBETimer” and then follow the previous step again
3.Then change some of the value data to anything else (delete some, add some letters,just change it!Now close out Regedit).
5 . Go to Start > Run > “%systemroot%\system32\oobe\msoobe.exe /a” (again,without quotes)
6. Then you will get the activation screen,and then follow the following steps
7.Click on “Yes,I want to telephone customer service…” and click on “Next”
8.Do not enter anything in the other boxes which ask for some crap !.Just press “Change Product Key”
9.then type in this product Key”JG28K-H9Q7X-BH6W4-3PDCQ-6XBFJ” into the box and click “Update”.This Key has not been blacklisted by Microsoft yet so any user who has installed this key will get a full and genuine version of XP !
10.Ignore rest of it and close the box after that click here to validate your windows.
You can Validate your XP and check whether it is genuine or not !It will surely be genuine