Ubuntu is free, as are most of the programs for it, you just down load it from them, the latest one with long tern support is 14.04 LTS http://www.ubuntu.com/
You need to know what you are doing IT wise if you are going to install it alongside windows, there are tutorials out there if you search for "ubuntu dual boot".
If you reduce windows size on the HDD you need to turn it on & off a few times before doing anything else (I reduced W7 from 750gb to about 550gb on my laptop, & then Ubuntu installed itself into the space when I loaded it from a DVD I had burnt earlier, having changed the boot device to DVD drive first)
The other alternative is to buy a new hard disc drive & put that in instead of the windows one & go into boot settings before hand to change the priority boot device to your DVD drive, & then start it with a DVD in with Ubuntu on it that you would need to down load & burn to disc first.
To change the boot device you have to press F8 or F12 (it may say at the bottom of the screen momentarily on start up ) when you start up your computer before windows loads, that takes you into BIOS or its equivalent, its very primitive looking, thats where you can set things like Boot Device etc.