because if you didn't specify your windows installation to be "secured", you're most likely wiped it out...and if the existing windows installation covered your whole HD, the only way you got your linux installed (I guess) was either to remove windoze or resize the partition and add new ones...and my guess on this would be that you removed the whole windows. if this is the case, of course...post more info.
however, getting windows back goes like this: make some space for windows with some partition app that recognizes linux partitions (like ext2 or ext3 and so on), by resizing or removing them. after that either leave the space empty (I'd say do this) or add a new fat-partition (or ntfs?) there. then just reboot and if you didn't create the fat-partition (which I prefer not to do in linux, but use windows' own tools for it's own tasks), do it now with windows' fdisk for example. then just install normally I guess.