Originally Posted by carnageX
Unless the OS is on a different partition, then no. Formatting breaks the links to the files on the filesystem table to allow them to be overwritten.
If you want to actually securely erase a drive, then you'd want to use something like DBAN or Killdisk which formats, and then writes 0's to the filesystem as well.
It won't allow you to format the drive that the OS is on. It will give an error when you try to format stating the disk is in use.
IMO, best way to format is with a GParted LiveCD.
Maybe I'm old school, but we call that a low level format lol.
The easiest way to format is by using the option you're given during the initial setup of Windows. Pop the disc in, boot to it, and one of the first screens you're brought to is the drive setup screen. You can delete your partitions and create a new one (system will also be created), then carry on with installing the OS. This is the easiest way for any person new to PCs to format and install Windows. If you format your drive it deletes everything. There is no way of keeping your OS or restoring your OS to fresh "out of box" state without doing a system restore from a disc you might get from a company like Dell. Either way, a fresh format is still better IMO, because you don't get all the bloat.
Most computer enthusiasts don't agree with using the format, but for simplicity it's there. One of the best ways is to use Gparted Live as suggested. If you feel you're up to the task, by all means. Otherwise go with the standard Windows setup.
Also, if you want to make any future formats a lot easier, after you complete your format and Windows install I would use a program like Acronis to make an image backup of your fresh install. If it's a laptop, it makes it real easy because you can install the drivers and then on each image copy or "format" you don't need to redo all that installing. For best results on this technique you will need to clone your drive on a different system.