Flush with my success earlier in getting apps installed on the SD card now mounted as 'c:\SD Program Files' I installed a few things. I then hit a snag.
When you install apps using an MSI, the installation files get cached by Windows Installer. Steadily, c:\windows\installer gets bigger and bigger, so whilst my apps were no longer taking up space, the install files were (and some of those are quite large).
I now have an image of the Vista install so I'm becoming more cavalier. I wondered if I could move the Installer folder from c:\windows onto my SD card by copying it into the mount point, but still get Windows Installer to work as though nothing had changed.
So, first I used Robocopy with the /SEC option to copy the installer folder tree over. Next, I deleted c:\windows\installer and created a directory junction which allows the moved content to still be accessed as c:\windows\installer.
To create a directory junction, use the mklink command with the /J switch to create the installer folder in c:\windows and point it at 'c:\sd program files\installer'.
So far, it's working fine. I need to try a few more installs to be sure, though...