Recently a client called to say that he could not update Microsoft Office via the Microsoft AutoUpdate program, and that he could not access Microsoft's website via Safari. I did some digging and it turned out that his iMac was infected with the OSX.RSPlug.A Trojan Horse.
I looked at the DNS numbers in the Network Preferences and sure enough, they were in the Ukraine! Followed the steps in this article, I was able to remove the Trojan Horse. I was then able to update Microsoft Office and access Microsoft's website.
Please read this article, if for no other reason it gives a good overview on how to protect your Mac.