Go here:
http://www.lavasoft.de/At the bottom on the right you will see -
Current Definition File:
SE1R52 30.06.2005 (this will differ)
Click it, then on the next page about half way down you will see -
Latest definition file: SE1R52 30.06.2005 (this will differ)
Click that to download the file.
Once it has finished downloading you will have a compressed folder, double click it and you will see a file called defs.ref.
Navigate to the drive that you have it installed on which would normally be as follows
(C:)/Program Files/Lavasoft/Ad Aware SE Personal (or whatever version you have), and drag the defs.ref file into there.
*edit*
IF ANYONE WONDERS WHY I POSTED AFTER VANGUARD SORTED THE PROBLEM, IT IS BECAUSE I READ HIS POST, BUT BY TIME I HAD TYPED ALL THIS UP AND POSTED IT HE HAD ALREADY POSTED AGAIN.