Analysis ToolPak

The following line may not work with non-English user interfaces since the name might have been localised

Addins("Analysis-ToolPak").Installed = True 

Calling Analysis ToolPak Functions

There are two ways you can call these functions in Excel 2003

value = Application.Run("MRound",10,3) = 9 
value = Application.WorksheetFunction.MRound(10,3) = 9 

In Excel 2007 you should only use the second method.
The first one will generate a run-time error

When a Com Add-in is loaded

Add-ins remain loaded even after they have been unchecked from (Tools > Addins) dialog box.
For more details about the VBA equivalent to some of the Analysis-ToolPak functions