I agree with Hoodie, a system restore (from safemode, hit F8 as your computer starts up repeatedly to get to a boot menu for Windows and select Safe Mode w/o networking) is your best bet.
If a system restore fails you may have to reformat the computer, a corrupted registry is never a good thing.
Be careful that you don't system restore to the date you got the virus, otherwise you risk the chance of restoring the virus! (Its a very unlikely chance but its not too far fetched if it touched the system folders, also some viruses /purposely/ infect system restore points so you can't just do a system restore to get rid of it...)
Basically
1) System restore
2) Backup important stuff and reformat (failing step 1)
3) Antivirus, Antispyware, and a decent firewall so it doesn't happen again
No matter what happens, heed step 3...
Common Sense 2009 is not a good antivirus
Also it sounds like your user account wasn't password protected if you weren't seeing a logon screen before... don't underestimate the importance of a strong password!