First post here from a lurker, a fed up with Nikon user, not buying another, wanta Canon 6d.
Post is intentionally maybe-ish being also unfamiliar with Win 8, still on XP SP3.
I would not be surprised if its because something in Windows 8 is grabbing access to the camera first. I ran into this issue on FoCal on Win7 when I had DSLR Remote Pro opened and it grabbed the camera first. Only one program can be accessing the camera at any given time. I am not a windows 8 user but I suspect there is some way of having Windows no opening access to the device itself. This is different than the CF card showing up as a drive letter.
Seeing files on 5D3 and control 5D3 are two different things. Accessing 5D3 file is via mass storage driver, which is a Windows building driver. Controlling 5D3 must go through Canon driver. If you have problem to use EOS Utility, that means the Canon driver doesn't work on your system. You have better check if there is any driver problem from device manager. If there is a problem, you need to reinstall the driver. If there is no problem, you have better contact with Canon for newer driver. You also can try the compatibility mode for your EOS Utility. Don't complain to FoCal because FoCal works via EOS Utility.
These two posts seem to me to be saying about the same thing, just saying it differently. Here's another take...
To browse the camera's files, it must be "mounted" as an external drive. "Mount" is a common Linux concept, Windows doesn't call it that, but it still does it.
In XP, to "unmount" a device, go to the System Tray at the bottom right of the desktop, find the icon for "Safely Remove Hardware" and double click on it. As the "Remove Hardware" applet opens, it'll list all the currently connected USB devices. Find your camera on that list and click on "Stop" which will "unmount" the camera even though it is still physically and electrically connected. This may free up the camera for lower level access by the PC's OS to the camera's OS through the EOS Utility.