Reading through the Invision forums, I see this is a common problem/complaint, as this software search routine sucks compared to others. I don't know if it's option you don't have on, or if a later version you aren't up to, but thier search page offers a link showing the search options to get what you need, under the keywords entry field. It shows why my search didn't work - it needs boolean operators by default:
Use Means
apple banana Find posts that contain at least one of these words
+apple +juice Find posts with both words
+apple -juice Find posts with 'apple' but not 'juice'
apple* Find posts with 'apple' and/or 'applesauce'
"Some Words" Find posts with the phrase 'some words of wisdom', 'some words' but not 'some noise words'
http://forums.invisionpower.com/index.php?act=Search&f=#