It must be a fake antivirus that is forcing you to buy it. its a rogue spyware that mostley appears with diffrent names like ANTIVIRUS 2009, XP ANTISPYWARE, VIRUS REMOVER 2008, VIRUS RESPONS LAB etc. it has many diffrent names to trap users, these a days it uses ANTIVIRUS PRO 2010 and ANTIVIRUS 360. these all fake antiviruses are rogue spyware, a clone from ANTIVIRUS 2009.
Antivirus 2009 is an unwanted program, from the authors of Antivirus 2008 . (it has many clones like antivirus 2010, antivirus 360, virus responselab 2009, ultimate antivirus etc). These applications have resembling interface and "features". After stealth installation, Antivirus 2009 will show tonns of fake spyware\adware detection messages and offers to remove reported threats (after you purchase commercial version). But in real Antivirus 2009 is not a spyware cleaner, it's just an imitation of spyware remover. Antivirus 2009 can also slow your computer and cause system errors and crashes. Remove Antivirus 2009 using manual removal instructions (for advanced users) or removal tool. use manual removal guide
http://remove-fake-antivirus2009.flesh9.com