Firstly check that explorer is starting up:-
Go to Start, Run, type regedit and press Enter. In the left pane navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon.
In the right pane, make sure there is a REG_SZ named Shell and it has a value of "explorer.exe" (no quotation marks). If it’s not there create it.
To be prudent create a system restore point before making any changes to the registry.