I got around this issue by installing Windows XP mode and then loading the application into XP mode.
Then I connected the phone to the USB and then presented the USB device to the XP Mode client and then I was able to make changes to the 7921. I had to do this with my 7920 aswell.
Windows 7 will see it as an unknown device.
There are no plans to offer a USB driver for Windows 7 or Vista at this time.
Only Windows XP and 2000 are supported currently.
It's not supported but it works, but only with windows 7 32bit.
You have to start the USB-Install-7921-7925.1-0-2a.exe via right click "Run as administrator" in the administrator context (UAC!)
Also when you are logged in as an administrator.