I can't use my external HD

Question

I bought a WD1200U017 004 WD Passport 120 gigs october 10th 2007. It works great when plugged into the USB port of my laptop(Medion). But is not recognized by my desktop computer, running Windows xp x64 sp2. It doesn't even show up in my computer window. Desktop has asus m3a78 pro mainboard.