Microsoft just came out with a patch this past Tuesday to fix a huge flaw. It turns out your computer can be effected even with the Windows Firewall turned on. I think you are ok if you are behind a router but update Windows if you have not right now.
I am no way expert to explain the flaw but I do know what to do. Anyone with a Windows XP or Vista machine should update immediately. There is no know threat for this flaw but it wont stop people from creating an exploit for this flaw that can effect un-patched computers. UPDATE IMMEDIATELY!