now if you have installed your bootloader into a partition, and NOT into MBR (master boot record), this should work:
start some disk partition program, like fdisk, from your linux install...I think it should start (from command line) like
now if you use fdisk, it tells you to press m if you want help. press it. then press p to print the partition table - now you should know what partition your windows is in, as well as the others..you'll see an asterisk, *, which means a bootable flag - now press a to change bootable flag and you're asked of a partition number. press the number that corresponds your windows partition - if it's the first partition on the disc, then usually 1. now press p again to see the changed table..I guess you now have to asterisks. press a again and now choose the other partition that has * in front of it (not windows) so that it's removed. check with p that the asterisk is now in front of your windows partition and there is no other asterisk, press d and give your linux partition's number, press d again and give your other linux partition's (like swap) number and repeat this until you only have your windows partition left (you can check this by pressing p after each d+number).
when you're ready, press once more p and check that you only have your windows partition and that it is active. then press w to save the table, reboot your pc and be unhappy with windows (heh, just joking..) BUT if you missed something, did a mistake and don't know how to undo it, DON'T press w BUT PRESS q so the table isn't saved. then just restart the app and repeat the steps correctly...
after reboot, windows should load up normally and you should be able to use windows's own fdisk to add new partitions to it.
but if you HAVE installed your bootloader into MBR, I guess you'll need to do this: follow the instructions above. when you save the partition table and reboot, put in a windows's install disc, and boot your pc with it (it won't boot otherwise I think) and clean MBR with fdisk (was it fdisk -mbr or what? better check that out) because that's what you need to do to get your bootloader out and windows in...I haven't done this, because I always install bootloader into a partition to avoid problems..