Malicious software ("malware") is installed on your computer.
Make sure that your anti-malware software is running, then download the
latest signatures and run a full scan.
If you don't have comprehensive anti-malware software, that's like
driving a car without seats belts or air bags. Either way, you're
eventually going to get hammered. Install comprehensive anti-malware
software and learn how to use its features. A 'comprehensive' solution
scans for all types of malicious software in the background, on demand
and on schedule.
For now try scanning your system with /several/ of the better online
scanners, such as:
Kaspersky Antivirus (
http://www.kaspersky.com/virusscanner)
Panda ActiveScan (
http://www.pandasoftware.com/activescan)
Download HijackThis from
www.trendsecure.com. Run it, save a log, and
post the log at one of the many sites that support HJT, such as
spywarewarrior.com, bleepingcomputer.com, and temerc.com -- but not
here. Within a day, sometimes within an hour, you'll have one-on-one
step-by-step advice from a security expert on cleaning up any
infestations—or you'll have a clean bill of health from the volunteer
expert.
Even the best detection and removal software can't fix every malware
infection. If none of the above remove the infection, you may want to
show the computer to a professional.
---
Leonard Grey
Errare Humanum Est
Security Tips for Everyone, from PC Magazine
http://www.pcmag.com/article2/0,2817,2334856,00.asp