Are you running your Outlook 2003 in cache mode? I am gussing you are. Try this it worked for me.
"Since you can download new emails when disabling the Cached Exchange Mode,
the issue is most likely caused by some corrupted messages from the last
email that was showing up in the cache mode till today. To avoid the issue
reoccurs once you re-enable the Cached Exchange Mode, I suggest you exclude
these corrupted messages from the mailbox or Send/Receive groups.
However, it may be time-consuming to find out the exact corrupted messages.
One way is to copy/paste half of the emails in the "potential problem"
period to the local PST, and then enable the Cached Exchange Mode. If the
issue disappears, disable the Cached Exchange Mode, and then copy/paste
half of the emails from the specific PST file back to the mailbox, and then
enable the Cached Exchange Mode to test the issue again. In this way, you
may finally find out the corrupted messages and then delete them and retain
all the other messages in mailbox to fix the problem. Another method is to
create a temp folder in mailbox, and then copy/paste all the emails in the
"potential problem" period to the specific temp folder, and then click
Send/Receive -> Send/Receive Settings -> Define Send/Receive Groups -> All
Accounts, and then uncheck the box beside the temp folder under the "Folder
Options" list to exclude the specific temp folder from the current
Send/Receive groups. It may be somewhat efficient than the first method.
On the other hand, you can also copy/paste all the messages or just the
messages in the "potential problem" period to the local PST file
permanently, since frequently backup is necessary and can help you avoid
unexpected data loss. For more detailed information, you can refer to the
following Knowledge Base article. Your understanding is appreciated!
287070 OL2002: How to Back Up, Restore, or Move Outlook Data
http://support.microsoft.com/?id=287070
Note: This article also applies to Outlook 2003."
Hope that helps!
Unsolved 
