How do I get rid of a security warning?

My vista laptop is showing a security warning that states: Application cannot be executed. The file mcui32.exe is infected. Do you want to activate your antivirus software now? If i select no, then it reappears a few minutes later. However if i select yes then it expects me to buy the antivirus software. Is there any solution to this? Please help, i am in alot stress trying to figure this out.