You place it in Minecraft's "bin" folder (Start > Run and type in %appdata%/.minecraft/bin), replacing existing one. Or you can move the existing one to some folder, and then put the minecraft.jar in the bin folder.
I think there are some programs or scripts that allow having many different versions. For example I have such script in Linux, but it won't work in Windows. If you simply replace the existing minecraft.jar from 1.4.4, you will have version 1.3.2, until you (force) update the Minecraft again.