Go with Cyberkitten's advice. Run all three programs.
The only thing to be aware of is this: Norton may not play nice with AVG. So to get around that (as horrible as it seems) download AVG, install it, run the scan, uninstall AVG. For some reason, a lot of Anti-Virus programs do that. They don't like each other for some reason.
The other option is to run an online (since I'm assuming this PC is on the net) virus scan that will also remove it. The one here
works wonderfully. It'll ask you to install something or other and do it (it can't "see" your computer otherwise). Safe site. One of the other "big names" in Antivirus run the site
After downloading them all, update them. Run it ONE AT A TIME. If you run Spybot at the same time as Adaware your PC will explode (figuratively). They pick up things that the other has picked up but they also pick up things the other does not.
The spyware removers are all "allowed" on the computer at once, so long as you just run them at separate times.
Ease of use - here have the websites
AVG's site (also has a spyware remover for free)
Spybot - You want the "Spybot Search and Destroy 1.4"