I would just get an external hard-drive and back-up any important files.  Then reformat the hard drive on the computer.  Ensure a good antivirus and Ad-aware are installed after reformat.  Then scan the external when you hook it up.  Not very geeky, but I find it effective.

Be honest, your friend is you and you are borrowing someone's computer to ask this question for your "friend." 

Might wait a bit, until a more technological skilled person reads this message.