|Note: This problem can be caused by ‘malware’.|
Click 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 by clicking Edit > New > String Value then right-click it and key in explorer.exe then restart the computer.
If the Start button is missing, open regedit via Task Manager by pressing Ctrl+Shift+Esc then on the Applications Tab click ‘New Task’ at the bottom, type regedit.exe and press Enter.
To be prudent create a system restore point (%systemroot%\system32\restore\rstrui.exe in Task Manager) before making any changes to the registry.