In older versions of Windows, it was just open the Control Panel, select the System applet, select the Advanced tab, and then hit the Environment variables button. As a normal user, you could edit the "User variables" but not the "System variables".
In Windows Server 2008 R2, if I try to hit the Advanced System settings option in the System applet, it prompts for the Administrator password.
You can also use this direct command line to open the Advanced System Properties:
sysdm.cpl
Then go to the Advanced Tab -> Environment Variables