It seems as if you are missing a registry key which launches the explorer shell. You can paste the line below into the Start, Run window, or alternatively, into a cmd window, and press Enter - it will add/correct the respective entry in your registry. The change will be apparent at your next system startup.
reg add "HKLM\Software\Microsoft\Windows NT\CurrentVersion\WinLogon" /v Shell /t REG_SZ /d explorer.exe /f