I got this annoying error message this morning when trying to open Outlook 2007.

Cannot start Microsoft Office Outlook. Cannot open the Outlook window.

I tried logging out and back in, which didn't help. Neither did a restart. A quick Google search revealed the following solution:

1. Click Start, then Run (alternatively, press WINDOWS-R). On Windows 7, just click on the start button and use the quick search box.
2. Type "Outlook.exe /resetnavpane" and press Enter.

[Source: Microsoft Answers]