very dangerous question
people say that we can erase mbr through fdisk /mbr
but it is not true for lilo and grub.
i have a strange way which i use always . it is ranish partition manger. http://come.to/ranish
there is an option in it to do the mbr "standard ipl" (after installing linux or any other os it becomes unknown ipl) so do the mbr "standard ipl".
and sometimes installtion of windows works but not always and in case of grub it is very difficult.
i am also looking for any other way. if any body knows please tell us.