|Is your taskbar gone as well?|
If you start Task Manager (Ctrl+Alt+Del) can you see explorer.exe in the Processes list?
If not, try this:
Be Careful. Messing around in the Registry can make your computer unbootable.
If you are unsure, get someone who knows the registry.
Start Task Manager.
File -> New Task
After Windows load correctly:
Click Start -> Run
In the left pane, navigate to
HKEY_LOCAL_MACHINE \ SOFTWARE \ Microsoft \ Windows NT \ CurrentVersion \ Winlogon
In the right pane look for a key
Its value should be
If there is anything else, double click the word "Shell".
A box will open with the value.
Delete everything before or after "explorer.exe".
Click on OK.
Close the Registry Editor.
Restart your pc.
If your Shell = explorer.exe, then do not modify anything.
But I suggest you let a computer technician take a look.