It can be the result of the developers for that site screwing something up, it can also be an early warning signal for malware since you're using Internet Explorer.

Main thing I would suggest, is first scanning your system for malware with 2-3 of the major scanners out there. Then, if you don't find anything, or the more likely event of you clean it all up, I would recommend the immediate halt of using Internet Explorer. You have three choices now for a replacement. Something from the good folks at Mozilla such as Mozilla Firefox or Seamonkey, Opera's Opera, and Apple's Safari. Pick one, or all of them, and stop using Internet Explorer. It will be to the great benefit of your system's security to do so.