Unfortunately, Microsoft seems to have removed this feature for Windows Vista and above, so you need an XP or 2003 Server client to set the variables, but it also works for remote Windows Vista, 7 or 8 machines.
Open computer management (compmgmt.msc) and connect to your remote machine.
Then right-click the first line with the machine name in the tree and choose “Properties”. You should see the common XP System window, where you can change to “Advanced” and go to “Environment variables”. You can then choose user where you want to modify variables.