G Data FakeAVCleaner is a portable security tool that will check for the fake antivirus program "System Tool" on your PC, removing anything it finds.
You've probably seen the signs of System Tool while you're browsing. Typically you'll visit a page, and a pop-up window will immediately appear, displaying a warning like "Windows Security has found critical process activity on your system and will perform fast scan of system files".
If you click the OK button, a window opens that appears to show a scan of your PC. You're then warned that your system is infected, and the site offers a download to help out.
As long as you just close these windows, there's no problem - you won't be infected. If you download and run the executable, though, it'll claim more infections, prevent you running some programs, close others down, then demand your credit card details to put things right.
Sounds familiar? Then G Data FakeAVCleaner should be able to restore your PC to normal working order. Simply close all your regular programs, run the downloaded file, and it'll detect "System Tool", remove it completely, finally rebooting your system to finish the job.