It sounds like you did either a repair install or a parallel install. A repair install over writes the existing windows files- your used drive space will remain about the same. A parallel install installs windows to a new folder leaving all of the old windows files intact- when you boot the computer you will have a choice between the new and old installations, the parallel install will use an additional 3-4 gigs of drive space.
If it’s your intent to install windows and have nothing else on the drive, you should partition and format the drive, then do a clean install of windows.
If you want to preserve your old data, you could use the new install to repair the old install, then delete the new windows folder. At that time you should click the tools menu>folder options>view tab- check “show hidden files and folders” and un-check “hide protected operating system files”. You may have some hidden files and folders left over from the new install
If you choose to do a clean install, you should zero or wipe the drive, then partition and format using the
drive manufacturer’s utilities. That will clean everything off the disk, it will be “like new"
http://support.microsoft.com/kb/316941/EN-US/