Today we were trying to install multiple versions (or) same version of 32-bit and 64-bit Python in same computer running on Windows 7. After installation, we noticed that it is replacing the previous version after installing the second one. First we installed 64-bit then 32-bit and observed that the system contains only one version of Python that too the latest installed one.
While installing the second version of Python, select the Entire Features will be unavailable under Register Extensions (as shown in the screen-shot below).
Register Extensions -> Entire Features will be unavailable
PS: This will work for Windows 8 as well