If you are having playback problems with .AVI files on USB memory sticks, find out if you your port is USB 1.1
If they are 1.1 ports and you have an available PCI 2.1 slot you can add a card.
I added a Belkin F5U219t and it works very well, it is only a three port card (2 Ext,1 Int) but the chipset is high quality NEC.