We have been experimenting with strictly per user installation of Fiddler for some time. Here you will find a stable beta of that flavor. All extensions are also attached since the original ones won't work in this setup.
The per user installation cannot coexist with the all users one. So if it finds Fiddler for all users installed it will remove it prior to installing. It leaves no traces on the system after uninstall so switching between per user and all users Fiddler is a matter of simple uninstall/reinstall.