Connectivity issues with Windows XP

Using the Micro Pro (32U4 processor) as a MIDI-to-USB device (MidiUSB.h library) I have no problem working with Windows 7 - all data transmitted & correct.
With the same setup into Windows XP (I have old synths I like to use which won't work under Win7), & I get the transmission of two notes then nothing more.
Since my sequencer, synths, & any other programs work flawlessly with WinXP,
Is there a general incompatability with MidiUSB libraries or is it more likely to be the Micro Pro driver?

I would do a test with the Keyboard library:
https://www.arduino.cc/en/Reference/MouseKeyboard
to see if the same problem occurs. I don't think the result of the test will magically give you the answer to your problem, but maybe it will at least provide another clue as to what is going on.