The driver for most external hard drives (including the (Maxtor OneTouch III) is built in to Windows (Microsoft generic mass storage driver), provided you are running Windows XP or later.
Sometimes Windows fails to assign a drive letter for an external drive, which means it's not visible to the Windows shell, but it is visible to the "Disk Management" console, from where you can manually assign a drive letter to that drive:
Control Panel > Computer Management > Disk Management
In the bottom-right pane, find the ribbon which relates to the "missing" drive (it will have no drive letter, no device name, and usually says "Raw" for the partition type.
Right-click it and choose "Change Drive Letter and Paths".
Click "New"
Select a drive letter from the drop-down box.
Click "Next" or "Continue, or "OK" (can't remember which of those three appears in the dialog box but click whichever you see).