Developing Windows Phone 8 apps requires you to be running Windows 8 on your development PC. If your PC is running Windows Vista or Windows 7, you should consider developing a Windows Phone 7 app.
Requirements: Windows 8 or Windows 8 Pro with a 64-bit (x64) CPU, 6.5 GB of free hard disk space. 4 GB RAM. To use the Windows Phone 8 emulator your PC must have Windows 8 Pro or greater and a processor that supports Second Level Address Translation (SLAT).