If it's worth buying is purely subjective, but I had fun with it on the XBOX, I liked it despite not being as good as the first, in my opinion of course, but it has its moments, to be honest I do prefer Halo 3 over Halo 2, but yeah, the original is on the top, I'd say that if you liked the original I don't think that you'll suddenly "hate" Halo 2, you may not like is as much, but you may well end up loving it, who knows really.
As for XP compatibility, as far as I know it's only possible to install Halo 2 on Windows Vista, officially that is. But let's not be naive for a moment and let's just say that there is a way to make it work on Windows XP. If you want to buy it go ahead, the price must have gone down since the time it was released, and it has some fun action, it's entertaining enough in my book. At full price I wouldn't have said go for it, but as I said, if it costs like $20 or something then why not, because it ain't worth much more than that.