You uninstall the usual Windows way...

Start - Control Panel - Programs and Features
click PianoCheetah
click Uninstall

It will ask if you want to delete the whole PianoCheetahSongs directory. There may be some midi files in there you'll want to keep. If so, click the default button; Move them; Delete that top PianoCheetahSong folder and you're clean.
Or just click the other button to kill everything off right now.

But thanks for tryin' out PianoCheetah !!
It'll improve over time, so please try it again some day.