It depends what brand and how old your computer is. Most computers nowadays have a windows image stored on your hard drive that will reinstall windows after reformatting it. Reformatting will completely wipe your computer clean so make sure you have anything you want saved backed up. It will start again like the first time it started and will ask you to make a user name and password. Be careful though because some computers will require you to have a restore disk to reinstall windows and the company will usually charge you $20 or so for it.
On a side note, never play around with system config. I by mistake unchecked the wrong program last week and my computer wouldn’t start. I had to completely reformat the hard drive which is why I know I about this. (thank god my friend was able to back up my stuff using linux beforehand)