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.
Hellllp! My pc crashed and ive lost all my addresses out of outlook express section. Being completely and utterly computer illiterate i havent a clue how to retrieve them, or even if they can be retrieved! Any ideas? I shall just sit quietly in a corner sucking jelly babies as i slip into the depths of depression until i find an answer to this! 
Twas the latter - everything had to be reinstalled. Pain in the bluming arse actually. But hey ho, such is life! Ty for the swift replies, i will give it a go to see if i can do a sucessful retrieval! If i fail i shall go and live in a cave on my own forever!