hmm.....honestly ur problem seems to be too weird to be true....if he has hacked ur orkut account, then it means he has hacked onto google servers, and in that case google shud be more worried than u.

a simple test or question, can u try logging into orkut from some other computer. if that also gives u the same problem (which i think is highly unlikely), then mail google customer care about this issue. if it works on another computer, then try uninstalling and reinstalling ur web browser and see if that helps. Search on google on how to completely uninstall the browser u have becoz with firefox, simply uninstalling it using Add/Remove programs doesn't help.

Let me know if the above things help u or not. If not, then I would advise some more solutions.