Which version of Windows are you using? (2000, XP, XP-SP1, XP-SP2, Vista etc)
When did this problem start happening?
Is your copy of Windows up to date with all the latest patches from Microsoft Update?
There is a official Microsoft article on repairing Outlook Express at
http://support.microsoft.com/kb/318378/en-us?spid=2578&sid=45 - admitedly that article might look a little daunting, but if you need any help with it, please just ask, and we'll talk you through it.