If you find a new icon near the clock saying “Your computer is infected” and after you click it it starts downloading a new “antivirus” program, well you’ve got a virus. The “antivirus” program is that virus and those warnings are there just to make you download an even worse virus.
To kill the virus you will need ComboFix. Note that the latest version of this virus “knows” that ComboFix is out to get it, so first rename combofix.exe to something else (eg. notme.exe) and then execute it.
Get ComboFix from: http://download.bleepingcomputer.com/sUBs/ComboFix.exe
Don’t forget to rename it!
More instructions here: http://www.bleepingcomputer.com/combofix/how-to-use-combofix