Hi,
I read a few posts about the problem and tried them. I tried repairing, uninstalling and reinstalling. Still was getting the above problem. During installation the update ran once, but after the restart the update window was greyed out.
After searching and comapring with a working version i found that the files in "Program Files>Common Files>BitDefender>BitDefender Update Service" were getting changed as part of the first updation during installation.
(This installation was an upgrade form v8 to v9)
I checked the other system where the BitDefender is running fine and there are no changes in the files of "Program Files>Common Files>BitDefender>BitDefender Update Service" they are same as when installed and the update service is working just fine in it.
(This was a clean Install no prior version of BitDefender was on it.)
Maybe this helps in understanding the problem and sorting it out.