Just asking this question shows you arent ready for Linux. Users new to Linux must spend hours upon weeks for many years goggling how to do this or how to do that. Linux is nothing like Windows. Many Distros pretend to having a graphical user interface, but unless you only want to browse the internet, you will invariably find yourself typing into the terminal.
I'm only half kidding.
If you are unfamiliar with Linux, you will need to spend a LOT of time fixing that. That said, probably Linux Mint 11 is the most complete, and user friendly. If you don't have a special purposes cut out for your old machine, the stock Mint distro will probably work fine.
Good luck trying to image the install once you have it set up the way you want. All the options I found suck- and not in a good way. Possibly one could use Hiren's Boot CD to run Macrium Reflect under Windows to make a good image, but I didn't try that.
Installing software shouldn't slow down Linux, unless you're taxing the system's hardware requirements. In other words, Firefox with 100 addons, and some of the more advanced 3D games probably aren't reasonable on your machine.