– Start -> All Programs -> System Tools -> System Information
– Check for the “System Type”
x86-based PC: It’s a 32-bit computer.
x64-based PC: It’s a 64-bit computer.
For your information :
Difference between 32 bit and 64 bit : The main difference between a 32 bit and 64 is that 32 bit system has 4gb(gigabytes) of space for addressing means that the 32 bit system has a limit of 4 GB RAM to process data where as the 64 bit operating system has 2^64 bits of space to address and supports 16 hexabytes of RAM to process data.