I once had exactly the same problem on a training site I was using and am again using. In my case it was something to do with the hosting company although I don't remember the exact details. What I remember was this; one company took over another and as far as I could gather some information either wasn't or was improperly transferred. When I rang them I was directed to one company who directed me to another who then referred me to the original.

From what you have said I don't think it is spyware or a virus (although that's a possibility - antivirus programs are not always successful).
Best course of action: avoid re-formatting and re-installing for the moment (I did that and still had the same problems afterwards

Try to get hold of the admin of Orkut via another computer and tell them what happened maybe they can help (perhaps by providing a different link). Hope this helps.