Welcome to Windows2Linux!
To start with, we'll make use of Canonical's distro's:
Kubuntu (KDE ubuntu): http://releases.ubuntu.com/kubuntu/feisty/
You can make use of either the CD or DVD versions (an USB version is also available), all of which are 'live' distros.
If you have any doubts, try the live 'try-before-you-buy' option (which is a hullvalot more than MS offers you). Pop the live disk into you machine, reboot, set BIOS to boot from disk.
THIS WILL NOT SCRATCH YOUR EXISTING SYSTEM! (unless you install)
Multi-boot systems are available too.
Typical Ubuntu desktop setup has taken me as little as 15 minutes on a spec PC, and is ready to roll, with full office suite, security & tools, with minimal intervention; still need obvious stuff like security, language, etc, etc.
Use Kubuntu for low-end users to wean them off windows; most won't notice the difference.