If you're talking about the installation executable files - i.e. what you run when you install the software - you can do pretty much as you wish. I have copies of e.g. Office saved to an external hard drive and to a couple of DVDs as well, just for safety. Backing up/copying (there's really no difference) your programs in this manner is entirely legal and doesn't count as an installation. And, in fact, absolutely should be done.
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