I just experienced this issue myself. The problem (for me) was that my PC was assigning the kindle to drive D:, which apparently windows 7 did not like. Here is the fix:
(With your kindle plugged in)
1) From the desktop, right click your computer icon and click "manage"
2) click "disk management" in the left pane
3) in the right pane, right click "kindle" click "change drive letter and paths..."
4) click the "change" button and make sure the "Assign the following drive letter" radio button is selected
5) choose a new drive letter in the dropdown box to the right (I chose Z
6) Click "OK" and verify that you can now access the drive correctly.
HTH