When trying to install Expression Web SuperPreview or Expression Web to any other drive then C:, the install button is disabled.
This "feature" has bugged me since version 1 all the way through to it's current version 3.
On Windows 7 and 8 I would say install it as per normal to your C: drive, cut the directory to another drive, and create a Symbolic Link using the
On Windows XP it's a little trickier, because it doesn't come with the mklink command.
Symlinking out the directory like this works because as far as Windows is concerned, the files are still in the same place, so Registry keys and shortcuts are still valid, and at the same time you've saved yourself some space.