TBH you can install programs onto any drive and anywhere in that drive that you feel like, as long as the OS can see the drive they will still work.

But as 'Program Files' exists any way you find that most software devs insist on them being installed there, for the majority of programs on Vista & 7 this isn't a problem. But for older version of Poser as well as DS1 & 2 it was (and still is for some folk) a problem, as the main content/runtime was kept beside the program, so any time you saved your files would get shunted to your VirtualStore by DEP (Data Execution Prevention (XP users be glad M$ made a mess of it when they added it to SP2)) and the rest of the M$ nanny state security system. Iirc the main runtime was pretty much tied to Poser, so you needed to install it outside Program Files to stop that from happening, DS just needed it's content moved and linked back to.

Nowadays DS3 installs to Program Files while the supplied content gets installed to (My) Documents, and I believe that since P8 Poser has done something similar, at any rate it's main runtime is no longer tied to the program and can be put anywhere you feel like.

It's all down to personal choice, me I have DS3A in Program Files and Poser 6 in Program Files (x86), my content is in F drive where both DS3 & P6 access it.