I have a huge rectangle "WARNING Spyware detected on your computer" I can't remove. "HELP"?

Specifically I can't say exactly where (what site) this uninvited HUGE rectangle virus warning came from as My boyfriend was using the PC when it happend and He/we were on aware of its presence only after it was too late and He'd signed off the net and our history is of no help as we have it set to clear all data after each session.Which brings Me to needing Your hel as to how I can get rid of it as I can't find where it's rooted in our system other than it's HUGE presence smack in the center of our desktop with it's "Please activate your anti virus software to clean your computer" on waiting to be clicked on. The other distinguishing items of identification it's displaying areTwo windows of which each are displaying: 1)"WARNING! WIN32 Adware.Virtumonde Detected on your computer." and 2)"WARNING! WIN32PrivacyRemover.M64 Detected on your computre." and at the very top left corner outline frame it has "Windows Warning Message!" written Which is above the Big Rust color Rectangle box desplaying in HUGE letters "WARNING!Spyware detected on your computer! Install an antivirus remover to clean your computer" SOO! if any body knows what the heck this is and more importantly How I can get rid of it and can take the time to post your answer I'd really appreciate your help. Thank You! cE cE & Big D. in Reseda