Windows 7 Minimum Requirements

Microsoft recommend the following specifications for a system running Windows 7:

• 1GHz processor (32 or 64-bit)
• 1GB of main memory
• 16GB of available disk space
• Support for DX9 graphics with 128MB of memory (for the Aero interface)

However, it is technically possible to run Windows 7 with a lower specification PC with as little as 256MB RAM, 8MB graphics memory and a 500Mhz PIII CPU [Source]. This would not be recommended, but it demonstrates the scalability of Windows 7 compared to Vista.