I’ve not had any issues with grunt on several different windows 8 machines.
If you open the folder: C:\Users\[username]\AppData\Roaming\npm
Do you have a file named grunt.cmd
in this folder?
If not I’d maybe try npm install -g grunt-cli
again, maybe from an elevated command prompt.
If this exists and you have C:\Users\[username]\AppData\Roaming\npm
in your PATH environment variable then typing grunt from a command prompt should work.
Silly question, have you tried closing the command prompt and opening a new one?