The definition file on your system might be corrupted. To check, locate the (default) directory C:\program files\lavasoft\Ad-Aware SE\ folder. Delete the files: defs.ref and def.ref.old and try again

If the update halts after only five percent of it is processed, your firewall is most likely preventing the update. To correct, disable the firewall for the duration of the update or configure the firewall to allow requests from Ad-Aware to pass through.

Web update confirmation dialogs are suppressed. To correct:

Open Ad-Aware.
Click on the Ad-Aware Configuration Window (the gear) icon.
Click on Tweak.
Click on WebUpdate Settings to expand it.
Deselect Suppress WebUpdate confirmation dialogs.

You have inadvertently turned on the Use HTTP Proxy option. To correct:

Open Ad-Aware.
Click on the Integrated WebUpdate Tool (small i) icon.
Click on Configure.
Deselect the Use HTTP Proxy option.
Try updating the definition file again.

If none of the above is preventing the update, it is recommended that you try to perform a manual update.
Do as follows: Close Ad-Aware.

Download the latest definition file in ZIP format and save the file to a temporary location.

Unzip the contents of the file either with your favorite ZIP utility or the built-in support in Windows to the Ad-Aware installation directory (default path is C:\Program Files\Lavasoft\Ad-aware SE\).
Open Ad-aware.
Confirm that the latest definition file is installed by checking out the Initialization Status in the main program window.