You don't say which versions of the software you're dealing with. Personally I have Acrobat 6. When installing Reader 7 after having the full version of 6 installed it allows me to have it default to the Reader when viewing so I'll assume it's version 7 you're asking about. If not, let us know.
I suspect the best way to get there globally for you is via the Registry. By the way, if you're unsure how to follow these steps, you're better off not doing so. (That's my "Scary Registry Warning".)
First, change one user?s settings to default to Reader. Then browse to this Registry key:
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.pdf
Once you?ve clicked on the .pdf bit in the tree, click File > Extract and save a .reg file. Then apply the same setting to each user via that Reg file. In my cursory test this worked well but this was all on one PC, not on multiples. It?s a bit of a kludge, which bugs me, but Adobe doesn't make this easy to do globally.