After a few tryouts of other Linux distributions (and they make it easy for you to try them out without losing your Windows system) I'm using Bodhi, and I'm loving it: imagine a world where you decide what software to install or not, where help is community-based and actually helpful & friendly, where you can customise your system just how you want it, where you don't need to spend any time fighting viruses or defragging your hard drive. You need to be an inquisitive user, to want to get to know your computer and not be afraid to try something new or to get on the forums for help.
How to get Linux: Download an image file, burn to a CD or stick, and boot. If you like it, install. Instructions are always given. Try getting that with Windows! Did I mention the cost? £0:00 including VAT.