I need to re-install Outlook Express on a Windows 98 machine. Can I do this without losing all the saved messages, and contacts book (email addresses) stored on that computer now, or will re-installing wipe all that out?
Any help would be much appreciated 
It should preserve all your emails intact, but if you want to be sure, copy all the files with an .mbx extesion to another folder for safekeeping.
I’ve done this with outlook express 6 (should work about the same way for older versions, afaik):
To save messages: Go to Tools > Options > Maintainance and click on ‘Store Folder’. Copy and paste the rather complicated folder name into the address bar in My Computer. Copy all these files to a different folder.
Reinstall, then repeat the first part above. Paste all the files back into the folder.
To save contacts: Tools > Adress book. File > Export >> Adress book (WAB). Save this as backup.wab (or whatever), remember which folder you’ve saved it in. Later, Import this back.