It is quiet easy my friend....

You first have to know if the HDD has space left, if not, just resize as much as you need to install Windows..Let's say if you have a 100Gb, all BT, resize it depending on the Windows you want to install, since every windows has their own space needs. I want to install W7, tehn I resize it to leave BT with let's say 60Gb, and the rest (40Gb) for my windows 7. Create a new partition with the 40Gb space, with NTFS Filesystem. Apply changes. (If new to linux, use gparted live cd)

Reboot with W7 installation disk, it will see the empty NTFS partition (The Windows loader do not recognizes other type of filesystems like raiserFS, ext3, ext4, etc)..Just install it...

One point to keep in mind, the MBR will be overwritten by W7 loader, so you won't be able to see the BT5 partition...No Problem!!!!, easy...

Reboot with any ubuntu live dvd and recover the grub...Mount the BT5 partiton, bind permissions, install grub again (install grub /dev/sdx)...The next boot you will see the Grub with all OS available.

I've done this many times with laptops and desktops.

That is all....There is no magic tricks or graphical "one button fix everything", "Move mouse and click on button" things..

The next time, First create a map of partitions you want on your HDD and see what OS you want to install, leave other partiton just in case you want to add more OS or use as backup for your data...Start with the MBR eaters (Windows loaders) and then add the Linux OS (Grub)...

If you do it the opposite way, your MBR will be overwritten and again, you will have to recover the grub, but no problem....

Hope it helps....