First of all you need to download the software from Microsoft. If you type "windows vista remote server administration tools" into Google one of the results it returns will be titled "Microsoft Remote Server Administration Tools for Windows Vista", click on this link. Alternatively a direct link is available here;
You will need to have validated your version of window in order to download/ install it.
NOTE: If you are running the 64-bit version of Windows Vista then this is not the right link for you. You can either go to the directly link;
Or type "Microsoft Remote Server Administration Tools for Windows Vista for x64-based Systems" into Google for the correct link.
If you're unsure of which version you're running then go to the start button, right-click "Computer" and then select "Properties". You'll see something like this;
Look at the "System type:" value and you'll see what version of Windows you're running.
Once you've got the file install it (it's a standard KB update file, you will need Administrator rights on the machine you are using).
After it's been successfully installed go to the Start Menu > Control Panel and select "Programs and Features", then select the "Turn Windowsfeatures on and off" under Tasks on the left;
The "Windows Features" dialog box will be displayed, scroll down to "Role Administration Tools" (under "Remote Server Administration Tools") and select the the following items;
Click "OK" to make the changes.
In order to make finding these under the Start Menu a little easier right-click the Start Button and select "Properties";
Select "Customize ..." and then scroll down the list until you see "System administrative tools" and choose where you want the tools to display;
Under the Start Menu you will now see an "Administrative Tools" option, under this you'll see the new AD Tools you've just installed.
NOTE: Sometimes a reboot is required to pick up these changes!