If you want to use the default classic Windows 7 Start Menu in Windows 8, you can follow this registry tweak.

For current user only:
HKEY_CURRENT_USER\Software\Microsoft\Windows\ CurrentVersion\Explorer
For all users:
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\ CurrentVersion\Explorer

REG_DWORD – RPEnabled
1: Metro UI – Start Menu
0: Classic Windows 7 UI – Start Menu

Alternatively, you can use these apps:
Metro Switcher http://www.windows8tricks/wp-content/uploads/apps/MetroSwitch.zip
Or
Metro Style Toggle http://www.windows8tricks/wp-content/uploads/apps/MetroStyleToggle.zip