Go to Start -> Type msinfo32.exe
From the System Information page under System Type you will find as x64-based PC (64Bit Processor) and x86-based PC (32Bit Processor).
Screenshot shows that this PC has 64Bit Processor.
*www.lytebyte.com/wp-content/uploads/2009/08/SystemInformationonWindows7for64bitprocessor.png
For more ways to find please read this