i like to keep my root directory (C:/) clean - does it matter where i install the BIAB stuff?

BiaB doesn't have to be installed in C: drive. You can install in another drive, even a partitioned drive.
Don't install it in the C:\Program Files\ folder.

And, you can even split the program to put the main files on one drive and the RealTracks & TealDrums on another.

but C:\Program Files\ is where most (all?) apps usually go - what's the problem?

You will encounter rights and permissions problems. Trust us on this.

Right or wrong, BIAB has always installed into its own folder. It makes it possible to delete the folder rather than run an uninstall routine, for example.

Matt's advice is correct. The \Program Files\ and \Program Files (x86)\ folders may have once been the place for such things, but Windows Security enhancements that have been progressively introduced have meant that it's very likely that not all of the required privileges and access will be available to files stored in that location.

In addition, the Band-in-a-Box and RealBand installations do not write anything to the Windows registry making it relatively easy to run the programs from the external hard drive. A minimal installation will install very few files onto the primary drive.

There are a few items that will be written to the boot drive, but they are very tiny. Fonts, registration key, paths to the plugin, and the most recently loaded songs are some, but as I say, they take up almost no room.

ok, i'm convinced - i make great use of a programming environment called R, which also works best if installed at root.