You could try Start, Run and type sfc /scannow. This will scan for missing or damaged Windows files. You may need to insert your Windows CD. Or System Restore may fix it.
click here for shutdown problem troubleshooting from Microsoft.
Less drastic ways to shut down your PC are:
Ctrl+Alt+Del then choose Shutdown from the Windows Task Manager menu.
Right-click the Taskbar and choose Task Manager from there and then shutdown as above.
Start, Run and type shutdown -s