Philip,
You could try deleting the adobe illustrator preferences file and see if that helps.
It would help if you told us if you're receiving any error messages as well, or if you could describe what exactly happens when you try to open CS3 (does the dock icon bounce, then stop bouncing, etc).
Anyway, to delete the preference file for adobe illustrator, you'll find it on your hard drive under your home folder (if you don't know where that is, here's the full path:
Hard drive --> Users -->
username --> Library --> Preferences. There should be a file there named something like com.adobe.illustrator.plist
Delete that file and try opening illustrator again.
Let us know what happens, and please include as much information as you can.