I have the same issue. In the past, you were given the option during installation. Not now. Even if it was the case where lots of stuff still went to C:, it would still be 168MB less than what is put there right now. I guess I'll have to use a competitor's product unless somebody from Adobe has any suggestions.
I was in the process of uninstalling Reader while typing the above comment - now it's uninstalled.
How frickin' huge is the stuff put on by Reader? Free disk space w/ Reader installed was 334 MB. I just uninstalled Reader and now have 2.38 GB of free space on the C: drive. Uninstalling Reader 8.1.2 is the only change I made and it's given me back over two gigabytes of space. I did not remove or alter any other files. I was not expecting to see that much space returned, or even a tenth of that. Even so, the size would not be an issue if I could install to another drive.