Outlook vs Outlook Express as default mail client

I keep getting a message on reboot that Microsoft Outlook
is not the default mail client, which I think comes
because of McAfee Spamkiller V.5 (it's also not delivering
accepted email into Outlook Express). I've always used
Outlook Express. What's the difference? If I try to
change to Outlook to solve my McAfee problem, can I change
back to Outlook Express if it doesn't work or if I don't
like it?