When this happened to us, we spent a lot of time messing around, and then used UnHack Me. I'd only use it as a last resort, as it's been known to delete great swathes of files that're infected but that you still actually need. There's a free thirty day trial, and apparently even after that it keeps working (but we decided it was so brilliant that we bought it after ten days). When you use it, it gives you an evaluation of how likely a hidden program is to be risky, and where it comes from (and I also use this to be sure). It doesn't take long and it's pretty easy as long as you pay attention to what's going on and don't just blindly click "yes, delete this file".

We also use Lavasoft's Adaware, and when we're not using Norton, Avira antivirus.