Sorry to bump this from the crypt but I hit the same problem trying to install my Lachesis in Windows 7 x64.
Basically the drivers installed ok but no firmware update would work on it. The first time I tried it a balloon popped up saying that the device driver could not be installed, subsequent times this balloon message didn't appear. Having Device Manager open I could see an "Unknown Device" appear every time I tried to do the firmware update, and disappear once I disconnected and reconnected the mouse (after the update failed).
Anyway, here's what I did to fix it...
1) Open Device Manager
2) Try to update the firmware, the "Unknown Device" will appear in Device Manager and the firmware update will fail
3) Before disconnecting the mouse and plugging it back in, navigate with the keyboard (mouse won't work anymore) to Device Manager, down to the Unknown Device, bring up the context menu and select "Update Driver Software"
4) Select "Browse my computer for driver software"
5) Click "Browse", and then navigate to or type in where the Razer drivers are installed (C:\Program Files (x86)\Razer\Lachesis\driver64 on my system)
6) Click "Next"
7) Windows will install some driver called "Freescale USB" (or similar) for the Unknown Device
Disconnect the Lachesis, reconnect it and go to Step 2)
You should find the 2nd time around instead of an Unknown Device appearing, the Freescale USB device will appear and the firmware update wlil complete successfully.
Hope this helps other people!
