I'm installing Blender 2.74 but there is a copy of Blender 2.73a on my computer. Does the installer uninstall Blender 2.73a?
Not in the conventional sense but yes, it will simply overwrite the common files in the directory and you will get a new executable, new scripts if any were added etc. Since there is a major version difference it will create a new config folder for 2.74 while keeping the 2.73 folder. (I have a few old folders going back to 2.52!)
By uninstall you are asking if it will remove your settings, the answer is no. However, on installing the new version it will use the newly created 2.74 folder so it will appear as it did out of the box the first time you used it. To get your previous settings and scripts, you can simply press Copy Previous Settings on the splash screen. If you want to keep the old version separate, change the install directory.
So, you don't need to uninstall the old version, if the folder is getting bulky delete the older version folders as you don't need them. Same goes if you use the zipped portable version.
See my similar answer here Where is the startup file stored? for a bit more info.