A while back I installed Office 2007 Beta 2, but today I wanted to uninstall it as there were a few quirks of Outlook 2007 that were starting to bug me. Back when I installed Office 2007 Beta 2, I installed it over Office 2003. After uninstalling Office 2007 I attempted to open Outlook 2003 and received this error message:
Cannot start Microsoft Outlook. MAPI32.DLL is
corrupt or the wrong version. This could have
been caused by installing other messaging system.
Please reinstall Outlook.
I repaired the Office 2003 installation but that did not help. I also reinstalled Office 2003 but that did not help either.
Then I came across this forum post that had something that worked (the solution is the last reply in the post). Mapi32.dll is obviously located at c:\windows\system32\Mapi32.dll right? Well, yes, but unfortunately the error message is not specifying the correct file name of the dll in question.
The actual dll that is causing the problem is MSMAPI32.dll, which is located at C:\Program Files\Common Files\System\MSMAPI\1033. Simply rename this file to something else (e.g. MSMAPI32_OLD.dll), reinstall Office 2003, and Outlook 2003 should work perfectly after that.
I'm not sure if this solution will have any effect on other permutations of how Office 2003 and Office 2007 are installed. e.g. if you get the error about MAPI32.dll if you uninstalled Office 2003 before installing Office 2007 Beta, I don't know if this solution will help you.