I thoroughly recommend a series of articles showing how to create a live CD of Puppy Linux, a simple non-geeky version from the hundred or so versions out there:

http://www.telegraph.co.uk/connected/main.jhtml?xml=/connected/2007/11/12/ecrboot12.xml

(Click on "Bootcamp Archive" at the bottom of the page for subsequent articles).
You can use Puppy on bootup, and you can save your settings after you have set up email, etc. Everything is clearly explained in the five Telegraph articles. It can be installed on computers with only 64Mb RAM and a 100MHz processor. Your Windows OS remains untouched:

"[Puppy Linux] runs directly from a CD... makes no changes to the data on your computer's hard drive or configuration (unless you want it to), so it is completely safe and you don't have to wait for a crash to try it out. In fact I would encourage you to use it and get to know Puppy, it may even inspire you to have a look at what else the Linux world has to offer."