When you install the Windows 2003 Administration Tools on your Windows Vista computer, you get the following error when you try to use them:

MMC could not create the snap-in.

For a workaround to this problem, follow these steps:

1. Login to Windows Vista using an account with administrator rights.

2. Download adminpak.cmd and place it in the C:\Users\username folder where username = the admin account you are currently logged in with.

3. Click Start > All Programs > Accessories and right-click Command Prompt, and then click Run as administrator.

4. Type the command below:

C:\Users\username\adminpak.cmd

If you are running the 64-bit version of Windows Vista, type cd %systemroot%\syswow64 then press ENTER first.

Now you can click Start > All Programs > Accessories and right-click Command Prompt, and then click Run as administrator. and type the command of the tool that you want to run, and then press ENTER. Add the tools you wish to use.

Source:
You experience installation errors and compatibility problems when you install Windows Server 2003 management tools on a Windows Vista-based computer