I would agree that a partial installation may have
occured. Also, if Excel was setup to be "Install On First
Use" when office was installed, this may explain it.
Around my office, i have seen this situation occur with a
roaming profile. The systems are installed off of a drive
image. Then complete the installation after it's
started. Problem being that the person logging in doesn't
have the rights to complete the installation.
Just some thoughts to it.