During solving various problems in Microsoft newsgroups I have come accross to this problem many times.It asks the user repeatedly to activate his/her Windows XP though it has already been activated.
How To Solve:-
Start>Run>type in:"regsvr32 regwizc.dll" (without the quotes) >ok.
Start>Run>type in:"regsvr32 licdll.dll"(without the quotes) >ok. You should get a successful message for both and it should resolve the error message.
If the first method fails then:-
Start the computer in Safe mode by hitting F8 at startup.
Now navigate to the following key:-HKEY_USERS\.DEFAULT\Software\Microsoft\Cryptography\ProvidersHKEY_USERS\S-1-5-20\Software\Microsoft\Cryptography\Providers
Click on the folder representing each key and delete them. Now close the registry editor and restart the machine.