How can i fix my pc from software counterfeiting?

How can i fix my pc from software counterfeiting.?
whenever i on my pc after then a message displayes on my disktop which says that " ask for genuine Microsoft software, You may be a victim of software counterfeiting this copy of windows did not pass genuine Windows validation". I have a" DELL" pc and i am using "winXP" which i have installed on my pc for last 3 months. this is the third time i am receiving this message but after some time i didn't receive this type of message and my pc was free of errors like that.
I did nothing to remove or repair my pc from this message, automatically after sometime my pc was not receiving this message.But this time i have to do somethig so that in future i wil never receive this message again.Besause i think this time it will harm my pc when i change my display picture after 15 minutes my displayed picture automatically remove from my desktop and my desktop turned on "BLACK" and on that black screen it is written that "Ask for genuine Windows software you may be a victim of software counterfeiting. this copy of windows did not pass
genuine windows validation".I didn't understand what to do so that i will never receive this message .Is there anyone who can sought out my problem.