to repartition your hard drive with out losing your data you will need someting like partition magic, and even then there is a risk that it will go wrong and wipe you drive, for the price you can get hard drives for now I would say stick another hard drive in the computer if you have room in the case
I use a distro called MEPIS on my desktop, which I am on now, it's setup and install is painless and pretty much works with most stuff "out of the box". If you have a decent internet connection it can be downloaded for free from
http://www.mepis.org (it's about 680Mb) the download includes open office so you won't need your copy, but �5 for the book is worth it. there is also a very good and active community on the MEPIS site that will be happy to assist with any question you have.
Once you have burnt the MEPIS cd from the ISO you downloaded you can boot your computer from it and run the OS from the CD so you can see what it is like, these are known as live CD's. bear in mind though it will run a lot slower from a CD than it would from your hard drive.
the chances are that you will come across a couple of slight problems, probably largely due to being used to windows but I wish you luck with your first steps into linux.
Mark