The most popular Linux distributions (Mandrake, SuSE, Red Hat) all perform partitioning as part of the install, if you choose to let them do so. The trouble is that unless you have enough experience to troubleshoot the install process, you can run into problems and end up being unable to boot Linux
or Windows.
I haven't partitioned my own hard drive yet, but I've studied this on the Linux forums quite a lot, and the same cries for help always occur:
I can't get Linux to boot, I can't boot Windows now, etc. So before beginning the install process with a Linux distro, you should do a full backup of Windows, and have an emergency boot floppy available.
Another option is to partition the hard drive using a third-party app, such as Partition Magic. Then when you install Linux, all you have to do is install it to the partition you have already prepared for it. In the cases I have studied using this approach, results were usually good.
You generally don't need more than 10 or 15gb for a Linux partition.
The easiest solution, if your computer tower will accommodate it, is to install a second hard drive for Linux use; it doesn't have to be a large one. This spares you the heartache of worrying about data loss and reformatting if you mess up the Linux install.
As for free Linux distros, you can download most, if not all, of them. You can get the ISO downloads for all distributions
here. The downloads typically take a half-hour or more, and should be verified by checksum after downloading. Then you must burn the ISO's to a CD, which you will use to do the install; a lot of people have problems with this step, including myself. I finally had to order some discs online because I was unable to burn the ISO's and get a bootable disc out of it.
You can find discs online very cheap, especially on eBay; the versions you'll want to get are the
community versions, which are typically only a few dollars apiece, and are basically download versions verified by checksum.
A great way to experiment with Linux without installing it to your hard drive it to get a
live cd version of the distro; Mandrake, SuSE, Linspire, Knoppix, Morphix and Linspire all offer this option. If you find yourself satisfied with the performance of the live CD, you have the option of doing an install of the full distro.