how can i tell windows to install things to a different directory by default?

Okay, you know the caveats of editing your registry directly. Do it at your own risk! However, here's what I found:

Go to Run.

Type: regedit

Navigate to:

HKLM\SOFTWARE\Microsoft \Windows\CurrentVersion

On the right side, find the string called ProgramFilesDir and modify it to the desired path.

One thing the link below didn't mention is if it will affect the paths for apps you already installed. This I don't know, as I haven't tested this solution myself. If you run into a glitch, just change it back.

One of the respondents in the forum below said:

The suggestion below works, but..... In XP, all of the annoying program subdirectories) (NetMeeting, Frontpage, etc) that you cannot get rid of in the default directory (usually c:\program files\), will magically appear in your newly-designated installation directory. In other words, MicroJunk will see your change, and act on it for its own purposes.....

And yet one more response....

This will also prevent some Windows Updates from installing properly. For example, Windows update KB905915 (and others) will fail before it has begun, indicating it was "canceled." In that case you will have to undo this change, apply the update, and then redo this change. It's not a particularly good idea (speaking from experience).