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.
Does anyone have an opinion on the paid version of Ad Aware? Is it that much better than the free version?