As to your first question, I would recommend that you install Lubuntu (instead of Ubuntu) on your 10-year old computer. Lubuntu is a variant of Ubuntu which is more lightweight and faster.
Related: What are the minimum system requirements for Lubuntu?
You can find more information about Lubuntu & download it here: http://lubuntu.net
As to your second question, it is possible to install Lubuntu AND Windows XP on your computer, and you can choose which one to boot each time your restart your computer. (That is provided you have enough available disk space to install both operating systems.) So, nothing happens to your Windows.
But the best thing is, you will also be able to try Lubuntu, without installing anything on your computer, before you actually do decide to install it. All you need to do is create a bootable USB drive or CD with Lubuntu, and boot your PC from it. Start from http://lubuntu.net, and if you have any more questions, do not hesitate to post them here.