This occured on a Windows 2008 x64 SP2 Terminal server. The error we were getting when users open outlook was:
Either there is no default mail client or the current mail client cannot fulfill the messaging request. Please run Microsoft Office Outlook and set it as the default mail client.
When you click OK, it then lets you into Microsoft Outlook.
First I deleted all sub registry keys that existed under:
Next I opened office 2007, clicked OK on the error, and ran Office Diagnostics.
Here it complained about 2 versions of outlook:
Sure enough on this terminal server when I went to the start menu, I saw both outlook 2003 and 2007. Some administrator had installed both for some reason!
After simply going into the Programs and Features in control panel, then changing the configuration on Office 2003, I just remove the outlook 2003 feature and left the rest of the office 2003 applications on the server. This resolved the problem.