There's a whole bunch of things that could be wrong here. But first let's start with some simple questions - which USB did you get? Did you get the USB-1 from Uniden, or something else? Did you download the latest XP drivers from the manufacturer's or distributor's website? Did you check Device Manager to see whether the COM port that got assigned was actually available? Did you assign the right COM port in the software you are using (and which software are you using)?
In general, the steps for attaching a USB device to a scanner are, from what I've read and been told...
a. Load drivers. Make sure you have the latest ones from the manufacturer or distributor
b. Check COM port in Device Manager to insure COM port is really available. Some devices, like Bluetooths, have a habit of grabbing a port and not letting it go even when it's not physically connected
c. Connect USB device to port
d. Start software
e. Ensure software can actually address the port you've allocated. Make the software look at the port.
f. Connect scanner
More details, please. 73 Mike