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.