It appears to be impossible to install 2.1 RC1 (win exe installer) on a Windows system that does not have a C:. When C: does not exist, the "Browse" button in the "Choose Destination Location" window pukes with the following error:
Error: window name "__choose_directory" already exists in parent
In this case there does not appear to be any way to choose an appropriate installation path. Developers should not make assumptions about drive letters.