They do not come with the OS. You have to make sure you have an OS you can install or you would have to pay $100 and purchase one.
---------- Post added 2011-02-27 at 01:10 PM ----------
That's a decent build. However...
-That's a GTX 460 SE, which is the shitty edition. A much better alternative for a budget build would be a GTX 460 768mb version.
-Paying $100 for a PSU for a budget build is insane. You shouldn't be spending more than $50 on a budget build PSU.
PSU I would recommend for a budget build:
http://www.newegg.com/Product/Produc...82E16817139019
It's not top notch quality, but your system is not even gonna use 300W at peak times, so there is no need to get a super high quality PSU. This particular PSU is decent quality, inexpensive, reliable, and still gives you a lot of room for future upgrades.