Contract law is a fun area to look at. Just about every part of a contract is there because of disputes, either to avoid them happening in the first place, or because someone got annoyed enough to dispute that point in the past.
So most contracts, these days, are actually pretty comprehensive. They're mostly enforceable, too. Obviously, illegal terms can't be enforced, but you'll have a hard time getting anything legal thrown out.
And now we come to the contracts most of us encounter most frequently: the End-User License Agreement. This is the box of shouty text that shows up when you install software, making you promise that you won't use the cheap version of Microsoft Word to make any money, and you won't use iTunes to run a nuclear power station (I'm not kidding; you can look that up!) and so on. We generally don't read these, and that can be to our detriment. For instance, some games come with software designed to prevent cheating in multiplayer which massively over-reaches, giving itself permission to provide every sordid little detail of your pornography collection to its parent company. And all because you've been conditioned by overuse of "Are you really sure?" to just click "yes" or the closest equivalent thereof.
But are these contracts legally enforceable? Are they, in fact, valid? Well, that's an interesting one. In earlier times, many software packages came with license contracts you agreed to by unsealing the box to read what you were agreeing to. To me, that seems distinctly off in terms of fairness. Not only is there no opportunity to negotiate, there's no way to back out of the deal once you've paid. Once the box is opened, the retailer won't take it back. The vendor knows this. These days, you're held to agree by installing the software, but again there's no way to read the contract until you're committed to the deal. You might, if you paid for the software, and had no opportunity to pre-read the EULA, be able to get said EULA thrown out as unconscionable.
This is actually why Electronic Arts, now generally known as EA, has taken to providing the license agreements for all their software on their website. They'll play it off as being public-spirited and wanting informed buyers, but it's really a way to make their contracts more likely to be upheld should it go to court. You lose the "I wasn't allowed to read the contract until I'd agreed to it" point. I should note that this strategy by EA is part of a multi-layered attempt to keep their agreements scaring you; the major plank, though, is strenuous effort to keep the agreements out of court. No software company wants to defend this before a judge, for fear of setting a precedent that would gut their leverage.
The other plank, of having paid good money for something, is lost if you didn't pay for it. Amusingly, the various Free Software licenses might prove more enforceable than any paid-for EULA.
My advice, though, if you've read this far, is to do your research. Find the agreements you'd be subject to by installing your new software, and decide if you'd be OK with every single aspect of them being enforced against you. While the likelihood is that at least some of them will be thrown out, particularly any which tread on the first sale doctrine, the clickwrap agreement is probably more enforceable than you think.