HEEEEEEEEEEELP! My laptop has a virus. My baby!

I don't know what to do. Currently using desktop computer, but in desperate need of help. Laptop will start normally and bring me to my desktop, but then whenever I click on anything, I get a message saying ''x.y file is infected''. I don't have an anti virus programme, and my computer won't let me surf the internet, instead a page comes up warning me that it's too dangerous to surf normally, so it's a bit of a pickle.