There is an "InstallRoot" associated with Excel in the Windows Registry.
If you go to
Run>regedit.exe
In the window that comes up Select
HKey Local Machine
Software
Microsoft
Office
I have an 8.0 and a 9.0 - select one
Excell
InstallRoot
It names the path to find Excel.
I have not a clue how to do this using java
.