I haven't tried this out yet (but I'm going to). I got this from the "Ask Marc Saltzman" column in the Costco Connection magazine.
If you're looking for a free downloadable dictionary/thesaurus that works with most programs, try www.wordweb.info/free
You hold down the control key and right click with your mouse on the word in question and a small window pops up with dictionary and thesaurus info, including Wikipedia links (which are not there when you use the program off line). The software is 7.4 MB and contains 150,000 root words and 120,000 synonym sets, and is available in several country versions.