Recently, I was helping Ashok in installing Outlook 2003 on his Laptop which had Vista Home Premium running on Windows Vista.
The setup program cancelled with this 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 found a technote from Microsoft which asked to run windows\system32\fixmapi.exe.
That didn't help.
I found some other posts on the net which advised doing this:
Locate the file MSMAPI32.DLL in Program Files\Common Files\System\MSMAPI\1033
Rename the file to something like MSMAPI32.XXX or MSMAPI32.OLD
This worked for us.