There are many options for you to choose from and many distros, the major difference between them being package management. Use the search feature here and also read about each individual distribution to see which may fit your needs best. Almost all can install side by side with Windows, giving you a dual boot system. If you've not visited DistroWatch
before, it makes for some excellent reading. Mandrake, Suse, and Fedora are easy to begin with--you may want to start there.