Explorer doesn't start automatically

Click start, type 'regedit' and hit <enter> to open the registry editor.
Expand the keys at left to reach:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon

Click on this key, then locate the 'shell' string in the right pane. Double
click it, replace the contents with just:

explorer.exe

Click ok, then close the registry editor and reboot.