I am having the same problems as in this thread.
I installed BitDefender 2011 last week and it was running fine. Yesterday my Virus Scan Module was unavailable. I read the thread above, uninstalled BitDefender 2011 and reinstalled it. Everything was back to normal. However, today I start up my computer and once again the Virus Scan Module is unavailable. So now I have no active antivirus program running on my computer.
And yes, I used the uninstall tool and removed the all previous versions of BitDefender. I also do not have any other antivirus programs running on my computer. I am running a Windows 7 64 bit system as was the other user in the linked thread above.