Sorry you have been having some trouble. I still have not got to the bottom of this but it certainly has effected a few users in the past. The problem is that I cannot reproduce it on my machine to attempt to debug it. I'm pleased to see that you have managed to follow the advice in the forum.
Since "=OVD(1)/UND(2) is showing 0.5" XLC.xll is installed - good.
If the toolbar is not showing then XLC.dll is not installed properly. Let's see if we can do it manually. Can you log in as administrator?
Follow these instructions to register manually:
1) goto start button and select "run"
2) type in "cmd" this will get up a command prompt window.
3) navigate to the xlc install directory (use "cd" commands usually "cd C:Program FilesXLC"

4) check using "dir" that two files are present XLC.dll and XLC.xll
5) register XLC.dll by typing "regsvr32 XLC.dll" into the command prompt window. - you should get a message saying you have succeeded in registering. It may give an error message and I would be interested in what this says.
6) hopefully you can see the toolbar when you start Excel.