After Installing VSTO on my PC, and trying our my first "Code Behind" project, I could not have access to the Excel designer. No controls were displayed in the Control toolbox and I could not any button or any type of control on the excel worksheet I was working on
The error message was a pretty scary one
"Excel Designer Could Not Be Activated"
After a bit of diggin on google I found this Post on the msdn forum, which helped me out sort the problem.
Among the different suggestions the one that worked for me was to reinstall the VSTO run time.
If you are developing solution for Excel 2003 and VS2008 this is the one you need
Microsoft Visual Studio 2005 Tools for Office Second Edition Runtime (VSTO 2005 SE) (x86)
or the latest version (up to date of publishing)
Microsoft Visual Studio 2005 Tools for Office Second Edition Runtime (VSTO 2005 SE) (x86)(build 8.0.50727.940)
If you are developing solution for Excel 2007 and VS2008 this is the other link
Microsoft Visual Studio Tools for the Microsoft Office system (version 3.0 Runtime) SP1 (x86)