Ok, sooo about an hour ago my computer was FINE! Now everytime I go to open an application it wont let me it says its infected. Even Itunes!! I click on an application (such as itunes) and it wont let me open it.

"Application cannot be executed. The file itunes.exe is infected. Do you want to activate your antivirus software now?"

Welll I dont have an antivirus software. I keep clicking no but it pops up all the time. Its all opened by a program called Antivirus Soft. I have no idea where it came from or how it got on my computer. Is there anyway to back up my itunes and pictures and stuff and just reset my computer???? Or anything?
Anything free that I could do Id greatly appreciate anyones help.