Rapid Environment Editor is a great tool for being able to get to the Windows environment and add, change, delete or update environment variables, both the system and user settings.
The place that I have found it of most use is when I run a lot of developer apps on a Windows computer, e.g. SQL Server & IIS. If you don't have a lot of compact directory paths, and use the default path names, you can run out of room for the path, and the applications don't work or function well. REE allows you to check the path for errors and fix them. If you put a wrong path in, it lets you know. It has context items to shorten (all 8.3 short-names) or return a shortened path to full extensions. Also variable expansion and cleanup of paths are also supported. I've also found REE useful in fixing QuickTime paths and variables issues.
Variables are flagged as read-only or updateable and the program allows either strings or expandable strings as data types.
This app is one of the first I put on any windows box.