It depends on the software, MB. Some will insist on removal of it's older version, some will install alongside, so you end up with two different versions of the same, and some will over-write the old version, and automatically update your files to the newer installation. For example, if you had Works installed, you can install Office along side it, as I have both on mine. If you have, say Office XP, and wanted to upgrade to Office 2003, I'm not certain, but I think you would have to uninstall Office XP first, as there's not really much point in running the two on the same computer, or it may just over-write the older version.
Ahh, Sandra, you beat me to it today!