1) On my Arm-Debian the default system has two users
login "user", with password "user" , and
login "root", with password "root"
I installed following the instructions from section 6 of Debian Installation guide.
2)Garbage through the serial port can be because of speed mismatch between GadgetPC\'s serial port and your terminal emulator\'s serial port settings. If you followed the "long" Debian installation (not section 6), there\'s a part where some lines must be changed in /etc/inittab to set the correct terminal and it\'s parameters for, make sure you edit that file correctly. If that isn\'t the problem i really wouldn\'t know.
3) The configuration parameters for the wireless usb card are in /etc/network/interfaces. I think configuration of this is fairly standard for Debian systems and some googling for topics such as "debian interfaces configuration" or "debian network configuration" or "debian WUA1340 configuration" should help you. Sorry to not be of much help in this, I was about to go do those searches myself!
(also there\'s a post somewhere in this forum about debian+gadgetPc+wireless+WPA, which maybe can also be of help)