Armaan i think during installation you would have chosen to use the max free space of the HDD and that resulted in Linux occupying the rest of the free space of your HDD....
As suggested by Gaurish, first allot a free space through Windows and while installing Linux make sure you select the partition created.
Secondly do not straight away delete the partition suggested by Gaurish as this may result in Boot failure. The GRUB would be deleted and you may have a system which will not boot....
So first thing u have the windows CD, insert it, boot through CD and repair the Boot record and then delete the partition in which Linux is installed....