"File or assembly name ESRI.ArcGIS.System, or one of its dependencies, was
not found"

...or...

"Unable to get installer types in the C:\program files\application_name\app.dll assembly. --> One or more of the types in the assembly uable to load."

So, you just installed a customization for ArcGIS Desktop and then you get this error!  What is going on?  The newly installed command, toolbar or extension can not find the ESRI .NET assemblies in the Global Assembly Cache (or GAC).  Normally these assemblies are automatically installed with ArcGIS Desktop buy ONLY if the ArcGIS installer detected the presence of the Microsoft .NET Framework.

The first step is to check if you have the appropriate .NET Framework installed.  If you have using ArcGIS Desktop 9.0 or 9.1 then you need Microsoft .NET Framework 1.1.  If you are using ArcGIS Desktop 9.2 then you need Microsoft .NET Framework 2.0.  Most installer (or setup programs) will automatically check if .NET Framework is present.  To make sure you can re-run the appropriate .NET install from the links in this paragraph.

Next you will need to install the ESRI .NET assemblies.  To do so you will need to restart the ArcGIS Desktop install and add a component called "ESRI .NET Support".  The following steps detail this process.

From the windows start menu click Start > Control Panel.

Double click on Add or Remove Programs.

Select ArcGIS Desktop from the list of installed programs.

Click the Change button to launch the installer.
(at this point you may be asked for the source media or access to a network folder that was used to install ArcGIS Desktop)

Select Modify and then click Next >. The following dialog will appear listing all the current installed components of ArcGIS 9.

Click on .NET Support and select Will be installed on local hard drive.

Click Next >

At the confirmation prompt click Next > again.  As the new files are installed the following progress dialog will appear. If necessary, the installation can be canceled by clicking the Cancel button, but this is strongly not recommended.

Finally click Finish from the installation wizard.

Done!