When starting Visual Studio you get a pop up which contains the following error:
The package 'Visual Studio Class Designer package' has failed to load properly. GUID {DD1683A7-5A4C-4234-81B3-A4CC91DBEAC9}
Close Visual Studio.
Now start Visual Studio with
devenv /log
Navigate to the log location, normally:
C:\Users\
username\AppData\Roaming\Microsoft\VisualStudio\10.0\ActivityLog.xml
If this log contains the following entries:
<entry>
<record>208</record>
<time>2012/01/17 15:06:00.310</time>
<type>Information</type>
<source>VisualStudio</source>
<description>Begin package load [Visual Studio Class Designer Package]</description>
<guid>{DD1683A7-5A4C-4234-81B3-A4CC91DBEAC9}</guid>
</entry>
<entry>
<record>209</record>
<time>2012/01/17 15:06:00.412</time>
<type>Error</type>
<source>VisualStudio</source>
<description>SetSite failed for package [Visual Studio Class Designer Package]</description>
<guid>{DD1683A7-5A4C-4234-81B3-A4CC91DBEAC9}</guid>
<hr>8000ffff - E_UNEXPECTED</hr>
<errorinfo>Catastrophic failure (Exception from HRESULT: 0x8000FFFF (E_UNEXPECTED))</errorinfo>
</entry>
<entry>
<record>210</record>
<time>2012/01/17 15:06:00.415</time>
<type>Error</type>
<source>VisualStudio</source>
<description>End package load [Visual Studio Class Designer Package]</description>
<guid>{DD1683A7-5A4C-4234-81B3-A4CC91DBEAC9}</guid>
<hr>8000ffff - E_UNEXPECTED</hr>
<errorinfo>Catastrophic failure (Exception from HRESULT: 0x8000FFFF (E_UNEXPECTED))</errorinfo>
</entry>
Now start Visual Studio with
devenv /ResetSettings
If this doesn't work, then repair or re-install Visual Studio.