USB Mounting to pc

When I connect my computer to my mpc the mpc says connected but my computer says "Mass storage device not recognized" I've read the manual and followed the directions. I've tried to re-initize the mpc but it did not work. I've also changed the os four times, 2.10, 3.06 4.44 and 4.5, nothing helps. I've also tried three different computers. Can anyone help me with this?
P.S. My cf card works fine, I can load and save with no problems, I use a card reader on my computer to transfer files the the card. I also installed a 40gig hard drive and still can't acess the hard drive or the flash card in windows xp.
Thanks, noob......
The card and hard drive are formatted and I'm using windows xp.

Try changing the cable maybe. Or try the jj os. Try it on another system. If that doesnt work take it back to where you purchased it to get it looked at