The free edition is for home use. Grisoft's professional retail product is generally sold for large companies, corporations, and the like. It's a great program that runs quietly. Even when having it totally disabled in the services and startup group found in the msconfig utility it suddenly came to life one day when a trojan was copied to the drive during a web search. That happened however while IE 6 was still in use. Too many vulnerabilities after the amount of time since it upgraded IE 5.5 users.
It's kind of like the football phrase of stating the best defense is having a good offense. While no one program will catch everything finding a good combo like adding Ad-Aware SE Personal another freeware, and Spyware Terminator have been good here. One will often catch what the other misses. Being familiar with several strengthens your main defense however.