Have you made any recent changes, installing updates such as your
anti-virus?

How to troubleshoot error messages that you receive when try to send and
receive e-mail in Outlook and in Outlook Express
http://support.microsoft.com/?kbid=813514

With some anti-virus (and anti-spam) programs, simply disabling the
e-mail scanning function doesn't really disable it.  It simply tells the
program not to flag any messages.  It still processes all the messages
and may still cause send/receive problems.  What you may need to do is
uninstall the anti-virus program, then reinstall it using a custom
install option and unselect the e-mail scanning component for
installation.