Because BitDefender has caused similar problems on other computers, please UNINSTALL it. (You can always reinstall it if it doesn't fix the problem.) Then follow the instructions below to reset a few things.

Re-register the Windows Update DLL with the commands below

Click Start, then type: then type cmd in the search box, and when it's displayed at the top of the search box, RIGHT click on cmd.exe, choose "Run as administrator" from the little menu.

When the command windows loads, type the following commands at the blinking cursor. Press ENTER after each command.

regsvr32 wuapi.dll
regsvr32 wuaueng.dll
regsvr32 wups.dll
regsvr32 wups2.dll
regsvr32 wuwebv.dll
regsvr32 wucltux.dll

Close the command window and reboot the computer. Attempt to run Windows Update

Hope this helps.