As in previous versions of Windows, you can still replace the Windows Explorer shell in Windows 7 with a different shell such as the command prompt or the Windows PowerShell command shell. To do this, open Registry Editor and locate the following registry entry:
Key: HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon
For example, you can change the Value to Cmd.exe or PowerShell.exe and then log off the computer and log back in again.
Note that you may even be able to run some third-party shell applications, though Microsoft obviously doesn't support the replacement shell itself (the vendor will have to support it).