Best not to do that.
There a way to install BT dual boot with windows7.(i'm using this method was)
Before install BT5 you must need a free space(you can use windows DISK Manager to compression space from your C: or D: and delete the new partition.now you have a free space on your DISK.)
After booting BT5 with USB or others. click install file on your desk.
During the installation process you need to manual partition.and redistribution the free space.(you can give 200M to boot and give 1024 to swap and all the rest to the /)
and netx you can manual the grub.(if you install BT5 the default value is correct,if you install BT4 you must be choose sda or hda,otherwise the grub does not work).
At now you done.
Forgive me broke english.