We have seen a recent rash of issues with Exchange MMC after April 11 updates, this is due to a .net change in the latest updates.
This issue is very specific to a system with
- .NET Framework 3.5 Service Pack 1
- .NET Framework 2.0 Service Pack 2
- Windows Vista Service Pack 2 or Windows Server 2008 Service Pack 2
- April 11 windows updates. (Specifically 2449742 or 2446709)
This problem occurs when the broken version of Hotfix 979744 is installed on your computer, and when security update 2449742 or 2446709 (part of security bulletin MS11-028) is installed in the affected environments, this problem generates the issues that are described in the "Symptoms" section.
You can apply the fix here to resolve http://support.microsoft.com/kb/2449742
Here is the KB that explains the issue http://support.microsoft.com/kb/2540222