Do you have VBA license installed? In 10, you need to install license separately. Request support for the license.
Is the machine that has ArcGIS 10 64 bit? If so check out the following KB article:
VBScript and JScript geoprocessing scripts do not run on a 64-bit Windows OS
gp.AddToolbox "C:/Program Files/ArcGIS/Desktop10.0/ArcToolbox/Toolboxes/Conversion