This thread should have been in the
Viruses and Malware board. Most likely a mod will move it there.
Anyway, what protection and operating system do you have? Firstly update and scan with all your protection apps. May I recommend AVG Free and Superantispyware.
Download and save
HiJackThis to a folder on your desktop and generate a log file, do NOT fix anything yourself, after this thread gets moved to the correct location you will get help to resolve the infection.
