If you already have an R6 installation on your PC then when installing R6.5 you do not get the option to select the program or data directories (the “Change” buttons are greyed out).
If, like me, you need to run several different versions of Notes then the “default” value may not point to your R6 directory but to something else.
You will then need to change (or remove) the offending registry keys that can be found in:
HKLM\Software\Lotus\Notes\6.0
I am not sure whether having concurrent installations of 6.0 and 6.5 will work correctly but the combination of 4.67, 5.12 and 6.5 seems to coexist OK.