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
-Proper nouns
-Related words
-Pronunciations
-150,000 root words
-120,000 synonym sets
-Look up words in almost any program

Check out WordWeb Free Version