creating .msi files f

i am running windows 2000 server on a rather large network and have multiple DC’s. I whant to use Group Policy to install patches and security updates to my users from one central location over fibre optic les2 connections. i do not have the budget to buy SMS server and do not whant to create .ZAP files aas they don’t work properly (does anything from microsoft?) i have heard about a tool that can repackage the application patch files as .msi extensions that can be installed through computer startup via group policy. Does anyone know the name of this tool and where to get it from.
RGDS
PCSTechnical