It's completely reversible, but might take some skill
Basically, we copy files into the install directory, set OUR settings in the registry, and (optionally) set geoShell as the SHELL in the registry. There's an article on the wiki which covers what you'd have to do to duplicate this by hand:
http://docs.geoshell.org/display/R4/Installing+GeoShell+from+a+Zip+File.html