A DNS trojan horse malware program package named "DNSChanger", also known as OSX.RSPlug and OSX/Puper has been causing Mac users headaches for about a year now. This malware is disguised as a video codec needed to view QuickTime movies on porn sites. The program changes the users DNS settings and automatically changes them if a user resets them.
The effect of the malware is very slow web browsing or a complete shutdown of web browsing altogether.
The first fix is to download DNSChanger Removal Tool-1.1 and run it. The link takes you to versiontracker, the most trusted updates site for software that runs on OS X.
Another method recommended by Apple is to remove QuickTime.xpt from Library>Internet Plug-ins. Check both the system Library folder located at the top directory of your hard drive and the Library folder located in your home folder.
As always, it's best to run all OS X updates using Software Update which is part of OS X.
Be careful out there!
Erik