If you wan't something different from XP, try Ubuntu, or it's KDE varient Kubuntu.
Ubuntu only needs about 4 GB of Hard Disk space, and can run on a minimum of 256 MB of RAM. In addition it is a completely free, open-source operating system, which offers in my oppinion even more than Windows ever could and ever will. It is a debian based Linux Distribution
Best of all, the software that comes bundled as part of the default installation as well as any other software you install via the Add/Remove panel on the Applications menu, as well as software you can find on the internet for Ubuntu, or Linux for that matter is free. Best thing is that none of them are trial versions that expire in X amount of days, they are fully functional and free to use for as long as you wish, and in many ways are even more feature and function-packed than their propriatary equivalents.
Ubuntu is also the most user-friendly Linux Distribution I have ever used before. It makes everything considerably pain-free and easy. The interface is also fully customizable (Not like in Windows though), you can skin it will all sorts of skins that people have made (Google Gnome Themes sometime), you can install and enable cool effects and animations for your interfaces. So if you ever wanted your windows to dissapear in a blast of flames, now you can.
Ubuntu will also allow you to install it right alongside an already existing Windows installation. It will allow you to resize the windows Partiton in order to make some free space to install Ubuntu on. Windows will try to run a CHKDISK after you resize it's partition because after Ubuntu has to resize it, Windows will think it's MBR is possibly corrupted,. Do not panic though, it will be just fine because at the end of the CHKDISK Windows will change it's MBR to reflect the new partition size.
You will notice a change with your Bootloader, as it will be replaced by GRUB which is a cleaner and more effective and functional bootloader to me than the one built in by Windows. Ubuntu, or Linux for that matter needs this bootloader to load, as the one Windows provides doesn't have the capability, and probably will never have the capability to load Linux.
You can also redistribute Ubuntu, Modify it's source code, behavior, etc provided you give credit to everyone who created that code, install it on as many computers as you want (Yes you heard me, you aren't going to run into licensing issues like you do with Windows), Download it legallyas many times as you want (Yep, no copyright issues from copying, downloading, or sharing it either).
To make matters even better, the ISO's of Ubuntu should or usually all contain a Live feature, which allows you to run the entire Ubuntu OS from the CD you burn and try it out before you install it.
As always, If you do decide to do this I advise you backup everything on your hard drive before you install Ubuntu or any of it's variations. Also, like every other OS Ubuntu and Linux in general will require a little getting used to.
You should also be aware that you most likely will need to have some sort of way of connecting through a wired connection to the internet in order to download utilities to get Wireless up and going, such as bfwcutter (For broadcom wireless cards. It allows you to cut and paste the firmware from your card so that it can write a driver for you that works. It will not from what I undestand allow you to ever get ahold of those firmware files from your interface so that you can distribute them. You might get lucky and have a wireless card that works right off the bat.
Printers are also something that you'll have to do a bit of tinkering to get to work, although alot of printers nowadays are a little more compatible with Linux.
Other than that Ubuntu is a great alternative to Windows as a whole. I successfully convinced at least half of one of the classes I am in at college to try it out and 8/10 of them have decided to stick with it as their OS. So, yeah if you don't have anything on that old computer, or even if you do, download Ubuntu, burn it to disk, and try it out through the Live CD feature (Current version is 9.10, although due to compatibility issues I am using 9.04 right now on my tripple boot macbook. I am actually posting this through Firefox from Ubuntu right now.) If you don't like it, then fine you aren't out anything other than some time and a CD-R, and you can still keep/or install Windows by itself. If you do like it then install it and spend some time getting used to it. Trust me, Ubuntu will treat you well, it's Linux for Human Beings!
Edit: Oh, I actually forgot to add this in. Ubuntu, and Linux for that matter as a whole, are completely immune to the effects of Windows Viruses. This doesn't mean you don't need an anti-virus, as you can still be infected by a Windows Virus. What I mean by this is that Linux wont be affected by them, as it's kind of like talking in Japanese to someone who only understand German, it just doesn't work, as they are completely seperate languages. Now, there are proof of concept Unix/Linux and Mac OSX viruses out there, but nearly if not all of them require you as the victim to purposely set up your computer in specific ways so that the virus can run, so unless you are really gullible you aren't needing to fear much.
ClamAV is a free-open source antivirus that is designed for Linux, Mac OSX (Called ClamXAV), and Windows (ClamWIN), it updates itself quite frequently and is very clean in terms of it's interface. It's only drawback is that it can only quaraunteen, remove, or if you so desire against best judgement ignore them. It detects Windows, Mac OSX, and Linux/Unix viruses.
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I live in the UK, and if i have to order online i can do
If you need any more info i forgot i'll give it to you. Oh yeah, I'm hoping to get Windows 7 working on it too
