All of the options that are greyed out in the free version are available to the user in Ad-Aware Pro, which makes it more configurable to your personal preferences.

It also includes the Ad-Watch real-time scanner,(as does the Plus version) and Process-Watch, which allows you to monitor and terminate running processes.

Scanning is no different than with the free version of Ad-Aware.

Having all 3 versions, my opinion is that the free version is suitable for most users. If you want real-time scanning, then the paid Plus version is all that a person needs. The higher priced Pro version doesn't really have much more to offer.