not quite understanding what you're asking. but are you asking....
you already have windows. you want to use debian and you're worried about the debian installing lilo or grub into the mbr? well if i remember correctly, you can always add the windows partition to either grub or lilo to get a dual boot menu when you boot up. not sure about debian, though, but you may even be able to specify this stuff at install where debian will find your windows partition and automatically put in a boot menu entry.