Most software packages let you decide where on your computer to install them to. This is doubly important for a map program that has 3 GB of data files. Garmin is hard wired to install only on the C drive, for some reason.
I didn't have enough room on my C drive for it.
The good news is that I found a free utility that allows you complete portability of which drive your Garmin maps live on, and it works well and is simple to use too:
JaVaWa GMTK, available from http://www.javawa.nl/gmtk_en.html
Now all my map data is on my E: drive, where I have a spare terabyte of room.