Just as an update I tried my Uno on a Windows Xp machine, following the instructions to the letter, but I get the same exact error on that system as well. I tried com3, where it first installed, and again on com5.
I turned off my Firewall, no luck.
I tried changing the baud rate on the driver - 4800, 9600, 115200... no luck.
I guess I will just wait for my external power plug to arrive and try that.