I'm a bit late to answer this question, I have used so many distros, currently xubuntu 14.04 64 bit, my laptop is hp pavillion g6 which is kinda old.
I'd rather criticize the answers given, then try to answer the question.
I don't recommend the Ubuntu family, the good thing is you get long time support release. the bad thing is the distro is bloated, even xubuntu, which is supposed to be fast, still slower than manjaro or crunchbang or any non ubuntu based distro on my machine at least. Sometimes it freezes, many bugs and so on. High possibility that many things will go wrong when upgrading.
the good thing is that you can ask on askubuntu, which is your number one priority. It's important to choose a distro that has a big community, especially when installing it on old machines, because you are going to have problems, no doubt.
I don't recommend Fedora simply because the release cycle is so short, upgrading is a bad habit when on old machine as I stated above.
As I stated above, I used archbang and won't recommend it to newbies. Archbang makes it insanely easy to install Arch which is a bad thing, arch repo doesn't deliver stable software, if you don't read the docs, you're system gonna break arch forum users don't help archbang users, archbang forums are inactive and you'll end up on your own, even unix.se users will have a laugh at you because you installed arch without reading the docs, and if you have time to read the docs you would have gone for arch, only the lazy choose archbang.
Recommending Archbang is like buying Lamborghini as your first car, so beautiful but so dangerous. Besides if you dare to ask an archbang question on arch forums, the admins will ban you, and tell you to contact them if you ever install arch so they can reactivate your account. Happened to me. They say Archbang is not Arch. And they are right about it, many differences between the two.
Never used puppy linux, can't judge.
I love crunchbang but don't recommend it to newbies, openbox is not for newbies, the forum users expect you to be good at Linux. you could install another DE, but then if you want to do so, why not going for another distro, crunchbang is meant to be used with openbox.
On old PCs you want either a stable distro that will never break and that has long term support, or a rolling release distro that is relatively stable so you won't have to upgrade it
I would recommend 3 distros, Debian, long term support and stable, maybe linux mint debian or any debian OS.
Debian Sid, it's a rolling release but it's relatively stable. There are many distos that are based on that distro
Manjaro, based on arch but more stable, they kinda test the software before pushing it to the repo, sometimes they wait for 2 weeks before pushing something to the repo so they make sure it's relatively stable. They have a friendly community that will not punish you if you did not do your homework: reading the endless arch wiki.