MBR was written by the Windos Loader...
You need to fix the Grub...
You have 2 ways to do it.
The simplest way is to download the Super Grub Disk 0.9 and burn it in a CD. Boot from it and select option to boot the GNU/linux.
This grub Disk boot your linux partition, so in this case should boot up BT5 (Ubuntu)
Then once BT5 is up, open up a terminal and type:
sudo grub-install /dev/sda (Do not place the sdaX, no partition number, just the hdd, it could be sda, sdb, depending in your Hard Drives in the PC or laptop. Yours? It should be sda. To check, type fdisk -l and see if is a sda or sdb...Remember, you are interested in the sda or sdb, no sda1 or sda2 or sdb1, etc. Just plain sda or b.
Once done the command before, reboot.
It might not see the windows partition, but once booted to your BT5, open up a terminal and type:
os-prober (it will search for OS in Hard drive, you should see the windows partition.
Then type: update-grub2.
Reboot again. You should see all OS, including Windows 7.
Try it..The other method is thru Live CD, but is a little more complex...