Try this, the simplest VBScript Custom Action:
Make a Type 38 Custom Action, which is a VBScript CA stored directly in the Custom Action
In the Target field (Or if you used the CA Wizard, you get a field to type code into) type
MsgBox "Hello From CA"
Put the CA into one of the Installation Sequences
You do need Scripting Runtime to be installed on the target machine for this to work
looeee