Hmmmm, if I understand the situation, your actions aren't really gone from your computer, they are just not showing up in CS3 anymore, right? You shouldn't have to re-install in the sense of copying saved files from somewhere else I don't think. I'm guessing you'll be able to get all your actions back, you just have to figure out the trick to do it.
I googled around a bit and found this:http://www.adobeforums.com/webx/.59b85abc
I'm not sure I've been any help...