Either tell me what I gain from using Linux, or I'm not even going to try it.
I can tell already that Linux is really not for you. Why? Because if you are the type who expects others to tell you something rather than seeking out an answer for yourself (which realistically would've taken all of five minutes), you wouldn't be able to handle using Linux as a desktop. This isn't meant to be an insult, it's simply a fact. Linux is (often, though not always) a very self-service platform and if you're not someone who's up to doing searches for information on your own, you'd likely find Linux to be only a headache. If perhaps this is an off-day for you or you just didn't find any worthwhile information on Linux. Here's one thing to know about it: it speaks for itself. There is no singular "Linux" -- it comes in a flavor (distro) to suit every individual and so what it offers is infinitely diverse (not kidding) and cannot be quantified in some "x is what you will gain from using Linux" sentence. In the end, it's really up to you and Linux has nothing to prove or sell you (other than perhaps customer support) so, if you're feeling adventurous and decide try Linux on an old computer, enjoy. I'd recommend DistroWatch for a hearty list of distros out there along with their popularity.
Never test for an error condition you don't know how to handle. -- Steinbach