Windows 95 question

We have old PCs Pentium II and Pentium I with 64 MB RAM and less than 10GB hard drives. Aside from Linux, installing Windows 95 could be an option so that these units could still be used. Do I still have to buy a licensed Windows 95 version?
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That is a good question, because I was under the impression that once Microsoft stops supporting the OS, we are free to use it. The only problem is that you might need some kind of key generator to make up a number that will satisfy the computer. However, if you want to update your version of Windows 95, 98 98SE or Millennium, there would be no place to go unless some University site has a complete update of these OS's. I have a copy of 98SE, but without a CD Key and I am at a loss as to how to install it without some kind of practical number/letter combination. I wonder if my 98 CD Key would work?