Web Browser Opens Stop-Malware-Site.com
Your web browser opens http://www.stop-malware-site.com (or other similar sites), with an offer to download an anti-virus program.
The anti-virus software offered is fake and malicious. Do not download it, or provide credit card information.
This redirection is caused by a rootkit infection which is very difficult to remove with traditional anti-virus software because it exists in the boot sector and is hidden from the system. Use Kaspersky's TDSSKiller.exe (direct link) to clean. (See also https://support.kaspersky.com/viruses/utility.)
This infection generally arises from other fake antivirus applications, so even after removing the rootkit there may be other malware on the system. Run a full virus scan after rootkit removal.
Also, some malware that is related to this infection is Cleansweep. This is hidden at C:\Cleanswepx.exe\cleanswepx.exe (note the folder name with the extension of .exe). This can be easily removed by mounting the volume offline, such as with the Repair option on the Vista/7 media.