I've been trying to install the game for a while now and keep encountering an error message

"Unable to execute file in the temporary directory. Setup aborted.

Error 5: Access is denied"

This happens even when I run it as an Admin so does anyone know how to get round this?

Do you have problems installing anything else, or is it Arcanum only?
I'm assuming you have either Win7 or Vista.

Usually caused by UAC so . . .
Turn UAC off before installing (see image), make sure you right click the setup.exe and click "Run as an administrator" when installing.

Good idea to install in a directory outside of Program Files. Create a directory (I like C:\Games). On the first dialog of the GOG installer, click Options in the lower left corner and point the install to the new directory.

Edit: set to "Never notify" in Win 7, uncheck in Vista BTW, turn UAC back on after install . . . =)

Sorry for the very late reply, I've tried turning the UAC off and it still won't work, and it doesn't even get to the installer, it comes up with that error message just after the file integrity check

Make sure your AV isn't getting in the way of the install process, maybe it's keeping some file from being copyed.