I want to design a MAX3420E-based device that does not conform to a standard Windows®︎ class. What do I use for a Windows driver?

There are two alternatives: 1) Write the Windows driver yourself. This is complex and difficult and recommended only for specialists. 2) Purchase a general-purpose driver. These typically consist of the USB driver and a companion library of C functions to access the driver. Drivers are matched to the VID (Vendor ID) and PID (Product ID) in your device descriptor. Microsoft has announced a general-purpose BULK driver for the USB in the upcoming "Vista" version of Windows.