Western Digital Passport Problem.

Hi,I have had alot of problems with my new Western Digital Passport. I am running Windows XP. When ever i connect it to my pc my pc dectects it but dont show under My Computer and dont have a drive letter. But once in a while the computer will dectect it and i have formatted it from FAT32 to NTSF. But i still have problems, i have looked at the WD Website and havent found much support. I have a older WD Passport (old gray big one) which was ATA connector. The new one black uses Serial ATA. The older one is dected once i plug it in my comp, but not my new one. The new HDD is not defective, i have gotten it to work on a computer (XP) perfectly. but my comp have no success, even after i reinstall windows XP.
Any suggessions? thx.