The issue with the updated definitions file has been resolved.
Please follow the instructions below to delete the corrupt definitions file, and restart your computer.

First you need to delete the broken definitions file.
Location: (2000, XP) - C:\Documents and Settings\All Users\Application Data\Lavasoft \Ad-Aware\core.aawdef
Location Vista: (Vista) - C:\ProgramData\Lavasoft \Ad-Aware\ core.aawdef

N.B. You will need to be configured to view hidden files and folders to find these locations, XP and Vista.

Then restart the computer and restart Ad-Aware.

Instead of restarting your computer, you can also restart the Ad-Aware service manually after you delete the corrupt def file.
1. Go to the start menu, click 'Run...' and type 'services.msc'
2. In the services window, locate 'Ad-Aware Service', select it and
click 'Stop'
3. When the service is stopped, click 'Start'
4. Start Ad-Aware again.

Kind Regards,
Jeff - Lavasoft

Edit. 20th March 09 - included hidden files N.B. Casey