So...there's no Buffalo driver (why?), however there's another thread about same thing and I did get it working on Win7 x64
You have to understand what you do and be careful. It took me a couple of hours with all instructions, and I know something about these things
I can't post drivers (rules, copyrights etc) but they can be made quite easily
Just remember you do this at your own risk (no warranties)
1) Download file Broadcom_4320_USB_x64_drivers.zip
First link has instructions (same copied here) second has the files
http://dumpytips.blogspot.com/2008/08/buffalo-wireless-adapter-wli-u2-kg125s.html
http://www.planetamd64.com/index.php?showtopic=19228
2) Unzip the file, there should be 4 files, one is BCMRNDIS.inf
if you don't see inf file, then change folder settings... show hidden files
3) open BCMRNDIS.inf file in notepad
add line after section [Belkin.NTamd64]
%BCM43GU_DeviceDesc% = RNDIS, USB\VID_0411&PID_00BC
and add this line on section [strings] at bottom..
BCM43GU_DeviceDesc = "Buffalo High-Speed Wireless G USB Network Adapter WLI-U2-KG125S"
4) save file on SAME directory (replace=yes)
5) insert your Buffalo Wireless-G High Speed USB Adapter
open device manager, it should show there with alert (no drivers..) WLI-U2...
6) Select WLI-U2... and update drivers (you have to confirm install..), select that directory where above drivers are
that's it, you can edit the settings in network card properties etc
"no drivers" should disappear from device manager and alert goes away. Reboot and repair connections might be needed after install...
and after that connect to your network
hopefully this helps
Juha from Finland