There is a very clear law that forbids any additional contract terms post the point of sale, so that if you go to a store, purchase a box with software in it and then go home to install it, when it pops up a dialog for you to "agree" on, you can just ignore it, nothing in that is enforceable at all. And no, small print text on the box that says you have to agree to terms in the software does not change anything. But that's not how software is sold anymore.
EULAs in general are not unenforceable, so long as they are presented before the sale. This is precisely why Steam (for example) now gives you the EULA before it lets you buy anything.