WordWeb is a one-click English thesaurus and dictionary for Windows that can look up words in almost any program. It works off-line, but can also look up words in web references such as the Wikipedia encyclopedia. Features of the free version include:
Definitions and synonyms
5000 audio pronunciations
65 000 text pronunciations
150 000 root words
120 000 synonym sets
Look up words in almost any program
Free. No SpyWare. No AdWare. No viruses. Works off-line.
Runs on Windows 2000/XP/Vista/Windows 7.
Download: WordWeb 6.1