Here is a fix I had to do to get mine to work,
Remove any memory in exess of the 2 gig basic. It is a problem that
microsoft is working to correct. A bug in the Outlook system files they
forgot about I guess.
I found the solution last night.
Call Microsoft and they will email you a hotfix for it. 1-800-642-7676
and tell them you need the hotfix for article #896253 which fixes the 4GB
Outlook 2003 bug. Since it is a bug, the support call and hotfix are
free. The reason they email it to you is they are still testing it and
don't plan to release it officially until they're done. In the meantime,
it appears the hotfix has to be updated weekly. They will email you a
link and two passwords for it. It was painless, no CD keys were asked for
but they did request personal information: name, address, phone #.
The hotfix is supposed to be installed post-SP1 but that didn't work for
me. I had to install it "manually", and before SP1, so if you can't get
it to work, try that order.