It's not "unusable".
WinKey+Break, Ctrl+Tab (three times), Alt+N, Alt+S, P, Alt+I, Ctrl+X, WinKey+R, vim (via ComEmu and Cygwin), ENTER, i, Ctrl+V, ESC, :%s/;/r/g, (do whatever editing you want), :%s/n/;/g, :%! cat > /dev/clipboard, Alt+Tab (until "Edit System Variable" window in focus), Ctrl+V, ENTER...
So easy even an Emacs user could do it! :P
@Libman Woah, why are you opening Vim again? :O
Can't you just do Start Menu -> Right-click Computer -> Properties -> Advanced system settings -> Environment Variables?
As in any OS, most things in Windows can be done by executing a command - and if they can't, they you can write the command yourself. You can execute a command from anywhere by hitting Windows Key and the 'R' key simultaneously (Win+R). When in Windows, I use ConEmu, which also adds some global shortcut key conveniences. Many Winbeard power-users don't even have a Start Menu!
The command to open the "System Properties" window is "control sysdm.cpl" (or just "sysdm" if your %PATHEXT% includes ".cpl"). Win+Break (the "Break" or "Pause" key on your keyboard) does the same thing in fewer keystrokes. But you don't need that GUI crutch to set environmental variables. There's the "setx" command, but that comes with Windows versions later than XP (or a "Windows Resource Kits" download) so I don't use it.
What some Winbeards like to do is to have backup files for parts of their registry, and to make changes by editing and re-importing that file. This can also be used to control all environmental variables. Such a backup for for global environmental variables can be created via:
reg export "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Environment" GlobalEnv.regUsually you'd have something like "c:\my\cmd" first in your %PATH%, and instead of modifying the %PATH% whenever you install something, you just create a little wrapper command there, so you can open the program with the desired settings (Window state, priority, etc).