I've installed Arcanum to c:\games\Arcanum on a machine that runs Windows 7 64-bit.

I've tried installing the unofficial arcanum patch UAP091225.exe

First I didnt run it as an administrator. This left me with an arcanum.exe file that had a size of 0Kb, so I had to reinstall Arcanum.

Then I tried running the UAP as administrator. Now it deletes arcanum.exe

Any clues as to the cause of this behaviour?

The patch does replace arcanum.exe with a modified version. It's possible that the new file was eaten by your AV software for some reason; try turning it off during patching or adding an exception for arcanum.exe. Or the patch didn't unpack properly, but that shouldn't be happening. It installs fine on my PC, same OS, same patch version, so I guess something else interferes.

Disabling my AV while updating did the trick. Thanks for the speedy reply!

As a matter of interest, which AV was it?