The 'scareware' fake-antivirus viruses that try to get people to pay for a piece of software to 'clean' their PC usually just take your credit card info, and run off with it. These can normally be removed with the aid of a boot cd (to disable the virus' startup entries) and a combo of MalwareBytes / Autoruns / Combofix / SuperAntispyware / whateverishotrightnow. My personal favorite boot cd at the moment is Windows Ultimate Boot CD - http://www.ubcd4win.com/
- I just use the 'EZPCFix' applet to load user hives and then delete suspicious start-up registry entries, and also delete all of the user's temp files which can slow down the scanners I'll use later. Recently I've seen a new flavor of FakeAV that doesn't show any registry entries that look off, but there will be a suspicious exe file in the primary user's "app data" folder... twice it's been "gir.exe" for example.
The worse kind are the viruses that encrypt your entire hard drive and boot up to a screen that says "Pay us or you'll never see your data again!" or something similar. From what I've read you can actually pay the ransom and get your data back sometimes, but I would be skeptical. The only thing you can safely do in those situations is restore from a previous backup.