Use DISK MANAGEMENT to remove the partitions on that drive (in XP) and then create it anew, format it and it will likely work. I'll leave it to you to research how Vista can do this.
I just bought a used, 120gb WD passport on eBay. The seller was a Vista user and didn't reformat the drive before sending it to me. I tried the HDD on my XP PC, and though the power light came on and the computer recognized it, I couldn't see it in Explorer. When I go into Disk Management, I can see it there, but when I right click on it all the options - including Format - are greyed out.
The drive works fine on my Vista laptop though. When I went into Disk Management there with the drive connected, I could see it and reformat it. So I did, but when I plugged it back into my PC it still didn't appear in Explorer.
I've also tried a fix I read in some other forum - disabling my 1394 connection and rebooting, but that didn't work either.
I need the drive to work on both systems so I can transfer files from my PC to my laptop. I'd appreciate any help.