While downloading a software it ask to choose 32 bit or 64 bit. I had also that confusion and after searching a bit I found the answer. To know your System is a 32 bit machine or 64 bit follow these steps. Click on the Start > All programs > Accessories > System Tools > System Information. On the right side find System Type.
If it is
- X84-based PC then it is 32 bit machine
- X64-based PC then it is 64 bit machine