I've had the same problem, before.
Trying at another time usually helps.
If it still won't update, try one of these:
Go to C: \ Program Files \ Lavasoft \ Ad-Aware SE Personal \ defs.ref
Open Ad-aware, and try to update.
If that doesn't work, another way would be to download the definitions file, and install them manually.
Delete the defs.ref
file (C: \ Program Files \ Lavasoft \ Ad-Aware SE Personal \ defs.ref)
Download the latest definitions file, in a ZIP file, from (This is a direct download)
Save the file to a temporary location.
When the download is complete, unzip the contents of the file to the installation directory of Ad-aware (Default Location: C:\Program Files\Lavasoft\Ad-Aware SE Personal)
Open Ad-aware, and confirm the success of the update.