I'd have to echo what rstanley has said!
There is only one GNU/Linux operating system, but there are many different distributions (or distros) available. And you can think of each distribution as a different 'flavour' of Linux. Each using a different mixture of package management systems, desktop environments and other software components; along with the GNU tool-chain and the Linux kernel at the heart of the OS. Essentially, they are all the same operating system!
The best distro is ultimately whatever works best on your systems and best suits your day to day computing needs. The only way to find out which is best for you is to download the liveCD/liveDVD images of some different distros and try them out for yourself. Using liveCD/DVD images on a CD/DVD or a bootable USB drive, you can try Linux on your PC without having to install anything. (So you can keep Windows intact whilst you are trying to find something you like)
Take a look at the distrowatch site (linked in rstanley's post) and see which distros are currently most popular. Take a look at the websites for those distros and take a look at any screenshots of it and see if any of the default Desktop environments available for it appeal to you. Once you have found a few distros to try, download the .iso's for them and try them out!
Once you have found a distro that you really like, the next step is to install it; either alongside Windows (dual boot, allowing you to boot into Linux or Windows), or completely over the top of it, so Linux is the only OS on the system!
