Greetings Pete,
The virus that you had is called a rootkit. TDSSKiller removed it. How you caught it I am not sure, it does not appear that you were downloading popular files like movies or music, that is a very common way to catch a pc virus. You may have opened an infected email or mistyped a url in your browser and went to a bad web site by accident if only for a second or two. That is all it takes. Someone else may have used your pc and gotten the infection for you.

You may also want to use the free versions of SpywareBlaster and Spywareguard from http://www.javacoolsoftware.com/ along with your Bitdefender.