Puppy Linux is great for the old machines. I use to boot up with a CD and save stuff on a memory stick. That way the computer I used was unaffected but still set up just the way I liked it.
You can actually set up a save file on your hard drive without messing up your Windows install.
Come to think of it, if your machine will boot from a thumb drive, you can use the same stick for both the operating system and the files.