The space you are referring to is the HDD (Hard drive) space, the OS deoending on which it is will be around 8GB of the space used and the rest will mostly be old files and programs, some of which can probably be removed, have a look in control panel and add/remove games and then remove anything that you are unlikely to use such as Games, and whatever else. MAKE SURE you know what you are removing before you actually remove it as it could cause you problems if you remove some of the MS built in stuff :)