HELP!! virus - dunno what to do

Hello,
I caught a virus somehow and its messed up my entire computer. i can still get on the internet as long as im using firefow (which is fine cause i dont use explorer) but anytime i try to open up any file it says the file has been corupt and asks me if i want to download "antivirus software" which i definatly dont trust so i always click no, and any time i go to download anything that might help it will pop up the same "corupt file" message and its really getting on my nerves. not only that but i keep getting the same two stupid nasty popups and security popups constantly and it is so annoying. i can still use the computer as long as its just for internet only, but idk how long it will be before it totally runs-down my computer. can someone identify this and gimme some help?