Hi tyros2009,
Which Windows 7 version do you have exactly installed? The Yamaha Steinberg USB Driver requires at least "Windows 7 SP1" (Service Pack 1).
The Yamaha Driver has nothing to do with your Lexicon Audio Interface. This either has its own driver or it is "USB Class compliant" so that no special driver is necessary.
It is also important that the SX600 is connected directly to the PC via USB, i.e. NOT via a USB HUB.
When installing the Yamaha Driver under Windows 7, it is important that you are logged in with an Administrator account. In addition, it doesn't hurt to start "setup.exe" with "right click > Run as Administrator" (instead of double-clicking).
In addition, the keyboard must not be connected to the computer via USB when installing the Yamaha Steinberg USB Driver. (In contrast to the installation of the Yamaha USB-MIDI Driver, which can only be installed if the keyboard is already CONNECTED to the computer via USB.)
P.S.
I have just installed the Yamaha Steinberg USB Driver V2.0.3 on a virtual Windows 7 Professional SP1 (under VirtualBox). The installation went without any problems, and after the restart there was no error message. Unfortunately, I can't test more for you because, as already mentioned, I don't have a device that is compatible with the YSUSB Driver.
Best regards,
Chris