Yes, most likely, Windows will "think" that your 302 is a mouse. This
is a bug called Crazy Mouse Syndrome.

Boot the computer with the 302 on and connected. The curser will be
jumping all over and all sorts of weird stuff will be happening.
Switch off the USB port. The computer will settle down.
Select Control Panel - System-Hardware-Device Manager
Click on the + next to Mice and other pointing devices
Click on "Microsoft Serial Ball Point" (what Windows "thinks" your
302 is)
Select "Actions" and then "Disable"
Close all windows
Reboot with the 302 connected
This worked for me - good luck. (This was written up from notes I made
a couple of years ago when I had the USB mod done. Hopefully, I took
good notes....)

Yes, you can use a USB to serial adapter rather than springing for the
USB mod to the 302. After 3 laptops, 4 USB to serial adapters, and
one PCMCIA to serial adapter, I finally
got a system that worked - except during contests and badge flights.
Then again, I am not a computer guru.... Of course, the only reason
why I went through all this crap is because my ancient laptop with a
serial port finally died.

When mounted in my glider, access to the rear of the 302 is
difficult. The solution is simple - remote the USB port and the
switch. Talk to the fine folks at Cambride before sending in the 302,
and let them know what you want to do. They can make the necessary
mods to the 302. I had the USB port and the switch mounted on the
instrument panel. It has worked flawlessly for the past 2 seasons.
Very simple and easy to use. Fire up the computer, connect the cable,
throw the switch, up/download using the Cambridge software. I keep
the USB cable in a zip lock bag inside a pouch attached to the seat
back. During contests, I also stash a net book (my dedicated "Soaring
Computer") behind the seat. (HP mini from Costco running MS Windows
XP Home Edition ver 2002 Service Pack 3)

As far as the cable goes, I am clueless. All I know is that it is not
a standard USB cable. When the USB mod was done, it came with the
correct cable. The only thing that I did was to take it to the local
computer store and tell them "Get me one just like this." One is
kept in the glider, the other, in my "glider box". Cheap insurance.

Hope some of this was useful to somebody.