have you tried another computer? do you have another cable? you also have to rule those things out as well.
do you have wifi or 3G model?
dropbox is even simpler than physically pulling out a cable and going thru the steps. if you just need to copy files to/fro dropbox is a simple fast solution. drop it and go.
further wifi explorer lets you connect thru your local area network.
i have seen people post here with similar issues in this sub forum so perhaps do a quick search back in feb. those types of questions, however, have slowed down quite a bit.
i totally understand your frustration... you just want the blasted thing to work!... if you can though, try another cable and /or computer. perhaps turn usb debug ON.
look in your device manager on your desktop (PC) and see if any usb ports have a yellow question mark to verify those work.
try a different USB port... do you have 1.1 ports or usb 2 or usb 3 ports?
lets rule out all of the above before we blame the xoom..