We got a new computer in a few days ago and it had a trial version of Office 2007 on it. Since it was trail version and we have the licenses for Outlook 2003, I had no desire to go the 2007 route just yet. So after uninstalling 2007 and installing Office 2003 I get this fun message when starting Outlook 2003 that reads something like “Cannot start Microsoft Office Outlook. MAPI32.DLL is corrupt or the wrong version. This could have been caused by installing other messaging software. Please reinstall Outlook.”.
I should also mention that when I went to Control Panel > Mail…I would get some random error message about not having enough memory. Either way, the first MS support article I found was of no use. And my first urge was just to delete or rename the Mapi32.dll file, but I want something solid to say “This won’t screw things up worse.” Which I finally found here (MS Support Article 926196)

The short of the fix is to go to C:\Program Files\Common Files\System\Msmapi\1033\ and rename MSMapi32.dll to MSMapi32.old (I also renames Mapi32.dll to Mapi32.old since it was referenced in the error message).

After you do this restart Outlook 2003, this will make Office rebuild the files and things should be fine.