I'm doing a dual-boot install with Windows 7 and following this guide:

I get to the part where BT says "Ready to Install", then I click the "Advanced" button. Here I want to change the location of the boot loader, because I don't want to overwrite the Windows boot loader. So instead of /dev/sda, I want to change it to /dev/sda5, the Linux root partition. This should be a valid choice, since it's listed in the drop down menu. However, whenever I select that choice, the "OK" button gets greyed out and I can't continue. It will allow me to select any of the other partitions except the one I need, /dev/sda5.

Does anyone know how to get around this? I really would rather not have BT overwrite my windows boot loader. I want to use the Windows boot menu and make an entry pointing to BT, as that guide shows.