Occasionally some malware will block AdAware from being able to connect to the Lavasoft update servers. It's particularly annoying, but easy enough to get around.
You can manually download and install the spyware definitions file yourself in just a couple of steps. First, you need to download the spyware defs file. Save it somewhere you can remember, and open up Windows Explorer. (Start->All Programs->Accessories->Windows Explorer.)
If you installed AdAware to the default directory, then browse to C:\Program Files\Lavasoft\Ad-Aware SE Personal. In there you will see a file named defs.ref. Rename the extension to .old or something if you'd like to keep a backup of the older file. Now simply unzip the newer defs file to the same directory, and that's it! You've updated AdAware!
IF for some reason you are unable to download the file at all, you can get it from a separate computer, copy it to a disk/CD and then copy it to the directory above.