Dens,
It is possible that your system is infected by the OSX.RSPlug.A trojan, a DNS changing Trojan horse as Dr. X mentioned. My suggestion is to Google for "DNS change Trojan os x". One of the first (or the first) entries is a Macworld article about the Trojan and how to get rid of it. Unfortunately, the directions are a bit technical. Let us know if you have further questions.
If you do suspect you have the trojan, don't type in your credit card or visit banking sites until you get rid of it. Better safe than sorry, right?
Bot