Well, if you want to stay 'current' get Windows 7
If you want stability get XP
As you noted, XP will become old sooner than the other choices, so for a 'new generation' approach I would suggest Windows 7 over Vista. Important to make sure your final input device is compatible with the chosen OS.

Another important decision is whether to go with 32 bit or 64 bit OS. The 64 bit versions of both Vista and Win7 have more issues with certain devices than the 32 bit OS does.

Latency doesn't generally occur because of the USB-MIDI convertor, but rather because of the device selected on the sound generating part of the midi chain. The exception in some PGMusic products currently is the EMU USB/midi input devices using XP OS. A few users have reported problems.

What do you use for midi sounds now? Just the MV 30? How does midi connect to computer currently?

I do not work here, but the benefits are still awesome
Make your sound your own!