The driver for all external hard drives is built in to Windows 7, therefore Seagate do not supply a driver as it is not required.
Windows sometimes fails to assign a drive letter to external drives, which means the drive cannot be seen in Windows Explorer. In that case, you can assign drive letter yourself. Make sure the drive is plugged into USB port, and also plugged into power supply if applicable:
Control Panel > Admin Tools > Computer Management > Disk Management
In the bottom-right panel are a series of horizontal ribbons representing each drive that's been detected.
Look for the ribbon which has no name, no drive letter, and it says "Raw" on the ribbon.
Right-click that ribbon and choose "Change Drive Letter & Paths".
Click "Add" and choose a letter from the drop-down box.
Click 'OK' and wait for the ribbon to update itself with the drive name and letter.
You should then be able to see the drive in "Computer" window alongside your other drive icons.