If you have empty space on your HDD you can use a partition editor to shrink your partition and install Windows on it, but beware that some vendors provide "recovery" or "restore" discs that wipe the hard drive including partition table and make a complete factory image on your hard drive...just like it was fresh out of the store. I use 2 hard drives, an 80 and a 120, with Windows installed on the 80. However, if you want to use one hard drive you could attempt this ONLY if you know the risk of what you are doing. A good partition tool is GPartEd found on many Linux (Gnome) distros.