Installing MS patches by Logon script

I am considering installing the latest RPC Patch using a logon script. We used to use the OS environment Variable to distinguish between operating systems when we only had 95 and NT but this is no use now that we have 2k and XP as they both have OS as Windows_NT. does anyone know of any other way to distinguish between the operating systems. I thought of All users path butits still the same between 2k and XP.If anyone has a sample script that would be even better