Let's do a quick check to see if it is a problem associated with XLC.xll.

Uninstall XLC and check that when Excel starts up that there are no error. OK

Now Do not install XLC.dll but go straight to PART 2 of the manual installation below. I am particularly interested in what happens at point 8.

PART 2 - Manually Install XLC.xll
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7) In Excel go to Tools>Add-Ins... This displays the Add-ins form.
8) From the add-ins form select the browse button and navigate to your XLC installation directory (normally this is C:\Program Files\XLC unless you installed it elsewhere). Select the xlc.xll and press 'OK'. This has manually installed xlc.xll
9) Close Excel and restart it again.
10) If all went well when you enter "=ovd(1)/und(2)" it will evaluate it as 0.5 and not give any "#NAME" error messages.