By purchasing the "Plus" version, you get an additional feature, "Ad-Watch".
Ad-Watch runs in the background similarly to an antivirus program.
It stops ad-ware and spy-ware before they have an opportunity to work.
Without Ad-Watch, any spy-ware or ad-ware will be active until you run Ad-Aware to remove it.
I still run Ad-Aware on occasion, but by allowing Ad-Watch to run, I have yet to find any spy-ware that has installed itself.