SQL Server Full txt Search - Thesaurus Entries
You should really consider upgrading to a more recent version, especially if you want to utilize some of the best parts of Full Text Search.

To your question, the short answer is yes.

From the documentation for 2005, you have two main features to customize in a thesaurus, Expansion Sets and Replacement Sets:


  An expansion set contains a group of synonyms. These synonyms are
  identified in code by "substitution" tags ( and ). Queries
  that contain matches in one substitution are expanded to include all
  other substitutions in the expansion set.


and 


  A replacement set contains a text pattern to be replaced by a
  substitution set. 


So if you place all three of your key words (IE, IE7, Internet Explorer) into a Substitution Set, then they will all trigger matches for each other. You can put as many substitutions in the set as you'd like - it's not a one-way definition for the substitutions.