Well I know you said win9x but if you want to use that P90 DOS will work for it just fine. you can use MPXF and MPXPlay to play mp3's. I personally wouldn't want to run windows on a P90....it might work...but it would be very slow. and boot time would be horrendous.
P.S. I'm running a P100 using DOS and my PC is booted and playing just our of the driveway.