first thing: PANIC!!!
Feel better now? Good.
By crash do you mean "the hard drive is completely bust and there is nothing left on it" or do you mean "I had to reinstall windows"?
If it's the first one, then you aren't in much luck, if it's the second one then you might be OK.
I'm assuming you're using Outlook Express 5 or above.
First search your PC for "*.wab" without the "s, this is the format your address book is in. If you find one you're in with a chance, if you find more than one you've got a better chance. If you don't then you might have lost it
If you find any files with a .wab extension copy them somewhere easy to find, like you desktop, then open Outlook Express and use the File>Import menu to import it.
There are some version changes between OE5 and OE5.01, so it might be a good idea to update Outlook Express to the lastest version.
Hope that helps.