Can I delete old versions of software?

The second part is easy: why? Who knows? There are so many possibilities,
and in general it'll vary from vendor to vendor. And of course knowing why
probably won't help us answer the first part of the question.

But I do have a suggested approach to cleaning up...

The general answer is yes, you can delete old versions that you know have
been superseded by newly installed upgrades and newer versions.

But, wait.

Before you run off and start deleting folders and files and what not, we
need to take care. The answer isn't always yes. Sometimes, for various reasons,
it's no.

So we need to be careful about what we do.

Here's the approach that I would take:

• Perform a full backup of your machine. And I do mean full.
  Make sure you're using a backup program or strategy that includes all files,
  registry settings and whatever else is on your machine. It's something you
  should have as a regular backup strategy anyway, but this is a case where we'd
  proactively take a snapshot before making major changes.

• Try Add/Remove Programs in Control Panel first. (that's
  Programs and Features in Windows Vista's control panel). Look
  specifically for the old versions of what you're attempting to clean up. Quite
  often if the old version has been left on disk, then it may well not have been
  uninstalled at all, and may still appear here. If that's the case, this is by
  far the safest way to remove it.

• Look for a program-specific menu item in your system's
  Start Menu. Specifically, look in the "All programs" menu, and
  perhaps even some of the submenus like "Accessories" or others. What we're
  looking for is once again an uninstall program that is specific to the version
  you're attempting to clean up. Much like Add/Remove programs, the old version
  may not have been uninstalled, and this, too, would be a safe way to remove
  it.

Now if neither of those options are available, things get a little
scarier.

• Locate the folder containing the old version. It sounds
  like you've already done this since you've identified that they're taking up
  space.

• Rename the folder. Perhaps rename "application" to
  "application-readytodelete" so you can tell later why you renamed it. This
  effectively removes it from system knowledge without actually deleting the
  files.

• Reboot. This is important because renaming the container of
  a file that's in use will often work without error - the file will still be in
  use. Sometimes the same is even true for renaming the file itself. It's not
  until you reboot and Windows attempts to locate the file again that an error
  might tell you it was in use originally.

• Run a test period for "a while". Depending on how you use
  your machine, and how pressing the need to clean up, this could be hours or
  days. When in doubt, I'd just let things sit as they are for a few days and see
  how it goes.

• Rename it back and reboot if renaming it in the first place
  caused errors, or any errors occurred during your test period. Based on those
  errors you can decide on how to proceed, but the obvious answer might be to
  leave it alone.

• Delete the renamed folder if no errors result during the
  test period. Chances are that the old version is no longer in use, and hence
  can be safely deleted.

Note that I said "chances are" in that last item. There's still certainly a
possibility that something you hadn't tried during your test phase actually did
still require what you had deleted.

That's why we started with a backup. If any step along this path fails, you
can always revert to, or restore portions from, that backup of your system that
you started with.