but i have installed linux already...
you will still have to supply a partition for M$ if there is not one already.
preferably place it 1st on the drive, in front of the existing lnx partition.
then install M$ with your M$ cd - the lnx will now fail to boot, as the M$ will
overwrite the mbr of the hdd.
you will now need to re-install grub, from the live cd of your current lnx O/S.
then, you will be able to dual-boot.