Why do i keep getting notes that say ,You may be the victim of softwear counterfeit?

It says my xp is what is counterfeit.What do i do?

Where are the notes coming from? Are they general popups or do they occur when Windows tries to download the automatic updates? Did you purchase your computer from a reputable dealer? Did the computer come with the Windows XP that is on there now or did a friend upgrade it or reinstall it for you using 'their' copy?

Sounds like a virus.