Square Bear. Here are a couple of things to try:
First, see what else is running when you open Express. There may well be a conflicting application (MS-Outlook for example), which is battling for control of your email address book. I'd suggest using MS-Outlook, rather than Outlook Express if you have it. You can quite easily and automatically import your address book and current emails into MS-Outlook, without having to re-enter the whole thing. An import wizard is included as part of the Outlook setup.
The other would be to delete unwanted mail and addresses, then compact the folders. Here, I recommend that you be ruthless. You'll be amazed at how much junk mail you'll be able to clear out. Some file corruption may have occurred.