I get asked this question often, but not often enough to remember the answer without a quick trip to Google.  By default, when searching for files containing a specific string, Windows XP only searches registered file types.  To change this behavior to search files of all types, run the following:

   mmc %windir%\system32\ciadv.msc

and then choose Properties from the Action menu and select "Index files with unknown extensions". 

It seems simple enough, but I can never remember it.  At least next time I'll know exactly where to go to get the answer.