To dual boot is simple, plus you are on the right track by already having windows installed. All you have to do is create another partition, the size is up to you (if you are going to use it as a full functional OS, I would suggest 30 GB or more, if you are just using it to play around (I have a 10GB partition with Ubantu BackTrak 4 on it to play around) To create your partition, just hit "windows key + r" type in compmgmt.msc, and go to "disk managament" under "storage" then shrink your C:/ drive volume (right click "shrink volume). Then you will have unpartitioned space. Then start up linux, aim it at unpartitioned space, and there you have dual boot. Since you installed linux last (which is what you wanted anyways) you will use the linux boot loader... Hope it goes well.
p.s. I would stay away from virtual machines unless you have a good CPU and a lot of RAM, running an OS within an OS is a struggle for many computers. And you don't want Linux using resources already being used by windows, it can get sloppy.