I'd like to transfer files to this Archos TV+ device from my Windows PC. (XP) Is it possible to connect the device over USB to USB like an external harddrive?
You may need Cross-Over USB cable (~$25) to have asynchronous file transfer. http://www.nextag.com/crossover-usb-cable/products-html
Alternately, preferably use router, or as a last effort a switch. This may help file transfer much faster.
Based on the device you have you should do the following:
You will be unable to connect the external hdd to the Archos TV itself, it does not appear to be a USB Host device, which is required for it to be able to see the files on your external hdd.