Firstly: If you have both Office 2007 and 2003 installed make sure that Outlook 2003 has been un-installed. I have seen both Outlook 2003 and 2007 installed at the same time this usually occurs when Office 2007 comes pre-installed and then you install Office 2003 additionally. If this scenario does occur then both Office 2007 and 2003 need Outlook to be un-installed and then the relevant one being re-installed.
Secondly: If you receive a message that you are using the wrong version of mapi32.dll or that the file is corrupted, you might have installed other messaging software after you installed Microsoft Outlook.
When you see this type of message, you can fix it by running a program in the Microsoft Windows System folder.
To run the program to fix this problem, in C:\winnt\system32 or C:\windows\system32, double-click Fixmapi.exe.
Thirdly: you can try renaming MAPI32.DLL to MAPI32.old and then opening outlook, it may ask for the Office disk when opening outlook so be aware you may need this.
Anyone of these 3 options should resolve this problem.