As noted above, ARC XT does work with Windows 8.1. The only stumbling block you'll encounter is the cable connecting the computer and the scanner.
The original USB-1 cable and many "older" USB to Serial port adapters use an HXA chipset. That chipset does not get along with Windows 8.1.
The USB-1 cables that Uniden started shipping this time last year have, according to Upman, have the newer HXD chipset. The HXD chipset does work with Windows 8.1.
I use ARC XT (the pro version) on my Windows 8.1 machine. I use a Plugable USB to Serial Port adapter, which I got from Amazon (because the newer USB-1 cables weren't available from Uniden at the time) and the programming cable that came with my 396XT scanner. Works like a champ!