If you install Windows 7 on another hard drive or partition on your computer system you might notice that the partition of the other operating system is not showing up in Windows 7. This happened to a dual boot system on a test laptop with Windows Vista being the initial system and Windows 7 being installed on another partition.
The reason for this is that the drive letter of the Windows Vista partition is not assigned in Windows 7 meaning the Vista partition does not have a drive letter and can therefor not be displayed in Windows 7. Here is how to fix that.
Click the Windows key, enter diskmgmt.msc and press the enter key. This will open the Disk Management interface that is displaying all connected hard drives and partitions. It should look similar to the image below, notice the missing drive letter in the first line.
All you need to do is to select the partition with the missing drive letter in the lower half of the screen (where it say Disk 0, Disk 1 and so on) and right-click it.
Select Change Drive Letter And Path from the context menu. Click on the Add button in the next menu and pick a drive letter from the pulldown menu.
A click on OK finishes the processes and the Windows Vista partition (or any other that was missing) should now be visible in Windows Explorer.