So to make this short and sweet I have a shitty old PC that I want to revitalize (Pentium D, 2GB DDR2) and I was wondering what would be a good Linux distro to throw on it and play around with. Also, do I have to format the HDD before I install anything else or what? I think it has Xubuntu or some shit on it. Thanks.
Xubuntu is OK, it's still Ubuntu Core based, which means not the most ravishing linux experience, but the Xubuntu community takes Ubuntu Core to a much higher level than Canonical's Ubuntu+Unity malware.
I would recommend OpenSuSE 13.1 for the moment, it's really easy to use and a great KDE experience in the main release version.
I always recommend Fedora, since it's the best of linux basically, and it's great in both the XFCE version (which should be the standard release version) and the Gnome version (more a Mac-like DE with emphasis on simplicity and integration, has hugely comfortable functionality and ease of use for getting work done, but for enthusiasts, it feels a little dumbed down).
Also a good place to start is Manjaro, which is basically "Arch improved". It's both downstream from Arch and upstream to Arch, and expands Arch with a lot of newly developed great features, but it isn't more secure than Arch, so it doesn't come as standard with SELinux enabled in the kernel, and it does come with a lot of concessions to privacy in that Flash, Steam and Skype are maintained on it, and it is made for use of proprietary graphics drivers (which is necessary for nVidia GPU owners anyway). Benefit of Manjaro is that you'll get support to the maximum of their abilities when you're using proprietary graphics drivers, which is a big nono for the major linux distros (because you can't offer support for hidden code basically).
On older hardware, Debian is also an option, if you don't mind some initial problem solving, because Debian never actually seems to work perfectly out of the box, but once it's all configured, it's very stable, but not adventurous. Debian Sid is an alternative. I would recommend using XFCE with Debian, the default Gnome on Debian is not that sexy.
The reality is, as you learn your way into linux, and forget your windows-habits, you'll start distrohopping anyway, and you'll figure out what you want to do with your system. Linux is completely different from Windows in every aspect, it allows you to take control of your computer and do things with it that are just not possible with Windows, and in the end, there are no two linux users that have the same install, every linux install is basically a custom install. Do not expect the world in the beginning, most users have a really slow linux start because they think windows and first have to literally start discovering their own computer, because Windows just turns any computer in a locked down microsoft console, whereby microsoft tells you what you can and can't do, and linux is the opposite, the focus is on unlocking all the power of the computer, making it function like a computer and not a console, and adapting to the user. The migration is technically not difficult at all (even though you can make it as difficult on yourself as you like), but mentally, it's quite a big step. Ideally, you should get help from other people in making the transition, talking about it, seeing it used, etc... and if you don't know any linux users in your entourage, you can always find the help you need on the many irc channels of linux distros and enthusiast groups, or on fora. Except maybe for the Arch crowd (not criticism, they've invested hugely into great documentation and as a consequence, have adopted a RTFM attitude, it makes sense), the linux communities may seem elitist, but they're really not, you just have to communicate with them, they don't know you skill level when you ask something, so not every answer may be suited to your skill level, so if that's the case, just tell them, and in one to two weeks, you'll be familiar with the linux lingo also and will be configuring your dream system like a boss.
I'm using arch, although I used to use Ubuntu, anyways manjaro is super easy to setup and run.
Pacman is easy to use also and be ready to tweak a lot and repair stuff but all part of the fun and the better Linux users here will help :-)