It depends on you, your usage of your system, and how OC(D) you are.
You used the clean install method to re-install; the
Windows.old folder is a copy of your previous Windows installation.
As such, it will contain the drivers, registry hives, system settings, Windows Updates, and everything else that goes in the Windows folder from your previous installation.
Much of that is disposable because they can be gotten again with the new installation (e.g., Windows Updates, .NET Frameworks, drivers, etc.) However, you may want to consider a few thing before deleting it since there may be some things you might want to keep.
Have you already re-installed all of your drivers? If you have some exotic hardware which you no longer have the drivers for and cannot find a copy, then you will want to copy them from the old installation.
Had you installed some software in the previous installation and don’t want to waste bandwidth to download the installers again? You may (depending on what it was) be able to find the installers in the old folder (e.g.,
Windows.old\…\Installation Cache, etc.)
Have you made changes to settings for Windows or your programs? The registry hives contain a lot of that information. You can mount the old hives and export the settings, then import them to the new system so that you don’t have to reconfigure everything manually. (This is usually an annoying, manual process.)
There could be other stuff in the registry (or files) that you want to keep, but again, that depends on your usage and how important it is to you.
One thing to definitely check before deleting it is to look through the user folders
Users\* to scan for anything you want to keep like files you’ve created/saved, settings for programs, and so on. (You can also look through
Program Files*, but there will rarely be anything you need to copy.)
Also of note is that the actual
Windows folder is not actually 44GB (or at least, it shouldn’t be). Windows uses hardlinks to make file entries that are really just pointers/shortcuts, and thus share space on the disk. You can find the actual disk space being used by looking at the Size on disk field in the folder’s Properties dialog: