As DeSmOnD dAvId suggested, password protect your PC. They won't be able to get past the welcome screen after that.

However, your mates might be shameless and might directly ask you to remove the password or something along those lines. Well, give 'em a blunt no and that's it! Why do you need to listen to them?
1. That's upto you actually.
2. Installing Linux might help initially but I guess they will figure out how to use it eventually. And don't you have a TV?
3. That WON'T work, assuming they are not a big bunch of dunderheads!