How do i clear my laptop? :(?

I have a dell inspiron windows 7 laptop and got a virus a few months ago, (i kow i should of fixed it sooner) i can connect to the internet, but it wont let me use it and says there are internet problems, which is wierd, i need to remove everything on my comp, clear it and restart, how do i do that? it costs �40 to repir in a shop, and i cant afford that atm :/ what do i do?! help please? :)