Proffesional would give you extra settings, cabablities like future proofing gthe product... but is not free. The Personal is teh same I think, and the Plus I belive is free? You don't really need all the professional features I don't think unless you are really worried, the free one should do fine for most peoples uses.
I advise also along with that to get Spybot - search and destroy which is also a very good spyware remover. This is so if one doesn't find something, the other will.
Find this at:
Hope this helps