NTFS? FAT32? Or Mac?
Go into Disk Management (both Vista and XP have it, but details are different) and see what it has to say about the contents of the drive.
I have just bought a new 160GB freecom 2.5"hard drive. I plugged it into my macbook first and it worked perfectly. Then i put it into my PC and it pinged and the device did not appear under 'my computer' I looked in the device manger and it is showing as a mass storage device and all is fine but i cant access it to put files on to it. What can i do, it came with no drivers etc!!!?!!