You installed the DNSChanger trojan or something like it, perhaps long ago, which redirected your DNS queries to a rogue server in Ukraine.
ISC Diary | DNS changer Trojan for Mac
If it's the original trojan, then apparently all you have to do is delete the bogus DNS server entries from your settings. Your DHCP server (most likely your router) will supply you with new ones. You may have to renew your DHCP lease to make that happen.
However, since we don't really know how those settings got changed, in my opinion the only sensible course is to wipe your boot volume completely. Back up all data first. After reinstalling the Mac OS, you'll be prompted to set up from a backup or a an existing boot volume. Choose to restore only your user data and settings -- nothing else. Reinstall all your third-party software from known-good copies.
Then you need to learn how to use the Internet safely.
Mac OS X versions 10.6.7 and later have built-in detection of known Mac malware in downloaded files. The recognition database is automatically updated once a day; however, you shouldn't rely on it, because the attackers are always at least a day ahead of the defenders. In most cases, there’s no benefit from any other automated protection against malware.
The most effective defense against malware is your own intelligence. All known Mac malware takes the form of trojans that can only operate if the victim is duped into running them. If you're smarter than the malware attacker thinks you are, you won't be duped. That means, primarily, that you never install software from an untrustworthy source. How do you know a source is untrustworthy?
- Any website that prompts you to install software, such as a “codec” or “plug-in,” that comes from that same site, or an unknown site, is untrustworthy.
- A web operator who tells you that you have a “virus,” or that anything else is wrong with your computer, or that you have won a prize in a contest you never entered, is trying to commit a crime with you as the victim.
- “Cracked” versions of commercial software downloaded from a bittorrent are likely to be infected.
- Software with a corporate brand, such as Adobe Flash, must be downloaded directly from the developer’s website. No intermediary is acceptable.
Follow these guidelines, and you’ll be as safe from malware as you can reasonably be.
Never install any commercial "anti-virus" products for the Mac, as they all do more harm than good. If you need to be able to detect Windows malware in your files, use ClamXav -- nothing else.
