Ok, I don't have xp , I am still on win 2000 professional, but if I recall xp is more or less the same , so here it goes;
1. go to settings -> Control panel
2. doubleclick system
3. doubleclick hardware
4. doubleclick device manager
5. select your soundcard under audio (just select no double click) then right click your mouse, which will give a popup.
6. select/doubleclick properties
7. select driver
8. select update driver
9. if there's a help screen coming up, select
next -> then select"display a list of the known drivers for this device ->next ->select show all hardware of this device class->new popup->select microsoft and voila there's your mpu 401 midi driver.!
(go thru the rightlist down it's there)
This is win2000 so it might be a bit different for win XP, but it usually finds for itseleve!
If you can't find it let us know.
In that case I'll cheque my son's pc which has win XP (He's gaming so I don't dare to interupt now.
)