An explanation from Microsoft as to why usbser.sys might be missing on your system:
Windows 8.1 and earlier versions
In Windows 8.1 and earlier versions of the operating system, Usbser.sys is not automatically loaded when a USB-to-serial device is attached to a computer. To load the driver, you need to write an INF that references the modem INF (mdmcpq.inf) and includes [Install] and [Needs] sections. Those sections are required for instantiating the service, copying inbox binaries, and registering a device interface GUID that applications require to find the device and talk to it. That INF specifies "Usbser" as a lower filter driver in a device stack.
[DDInstall.NT]
include=mdmcpq.inf
CopyFiles=FakeModemCopyFileSection
[DDInstall.NT.Services]
include=mdmcpq.inf
AddService=usbser, 0x00000000, LowerFilter_Service_Inst
[DDInstall.NT.HW]
include=mdmcpq.inf
AddReg=LowerFilterAddReg