Twice recently when I opened Outlook Express(6.0) several hundred messages were missing from my Inbox. I am running Windows Xp and use Butdefender Internet Security 2010 as my virus protection application. Can someone please tell me what the cause might be for this, how to fix it, and if possible how to recover the lost messages. Thank you!

There are a few programs specifically designed to recover OE mails however they all cost $$$.

Google

In the future you can backup your files as seen here:

How to back up and to restore Outlook Express data