Yes, just insert the windows CD and do a "clean install" .... it will do the format and everything else. To be safe, disconnect the HD data cable while installing.
http://www.sevenforums.com/tutorials/1649-clean-install-windows-7-a.html
http://www.eightforums.com/tutorials/2299-clean-install-windows-8-a.html
And you do not "necessarily" have to buy a new license if you change Motherboards
1. For Windows 7, if you have a "retail" version of windows you may install it as many times as you need as long as it's not used on multiple PC's at the same time.
2, If you have an OEM license, the activation checks a certain number of hardware hashes and if you change too many things within 120 days, electronic activation will not work. In the past cupla years, I have changed MoBos and CPUs (1156 => 1155 twice, 1155 =? 1150) once) and all three electronically activated without a hiccup. The change MoBo thing that everyone always quotes probably was more of an issue when activation 1st started with XP.... but changes have been made since. MS quickly realized that with chip sets and CPUs / sockets changing so fast, manufacturer's would be hard pressed to fulfill 3 & 5 warranty requirements if Windows wouldn't activate on the replacement board. So.....
If you replace a MoBo with the same brand / same class one, you'll likely not have an issue. If you go from a Asrock budget board to a high end Asus board, that may be a problem.