Try this :
Run "MSCONFIG" from the "Run" or "Search" box on the start menu.
Goto the startup tab.
Click "Disable All". Click "Apply" then "OK". Allow the PC to reboot.
If the start menu appears, it's a third party application loading when Windows starts thats the issue. You can go back into MSCONFIG and put check marks back to critical items such as anti virus, etc and you can narrow down what might be the cause.
To reverse it if it doesnt change anything, just do everything I said except click "Enable All" instead.