"This copy of Microsoft Office is not Genuine"....?

I purchased a pc some years ago with Office 2003 preinstalled which was working fine, but just sometime ago, I started getting a popup message called "Microsoft Office Genuine Advantage" that will pop up whenever I start office, and then it invites me to buy and validate a copy of Office 2007? Is this some kind of virus or something? Any ideas? Thanks

Can you ask the person from whom you purchased the computer whether he had genuine copy of Office installed on his machine? If not then you will have to buy a proper version of the Office suite from microsoft website and install it on your pc.