The MBR is only so many bytes long. The bootloader writes to these bytes and
points to grub in your /boot/grub folder. You can change the substance of
grub without disturbing the MBR. When you install another distro it will
overwrite the MBR and point to its own instance of grub instead of the old
one. You can in fact have several grubs. I have 7 of them installed and can
read them all with a text editor. But only three of them work. They link to
the MBR of each of my three drives.

When you remove Linux, you need to use your Windows CD to overwrite the MBR
to remove links to a non-existent grub. It will then point to Windows' own
boot loader. Otherwise you get an error.

My first drive has Windows and Sabayon. When I boot that drive grub will
load and point first to Sabayon, Windows and other distributions that I have
installed on other drives in order. My second drive has Kubuntu. When I boot
that drive its grub will be different. It will list Ubuntu, Windows and
Sabayon and then other distributions on my third drive. My third drive has
Sidux, Fedora, SimplyMEPIS, and PCLinuxOS. (I two instances of Fedora 12 &
13 installed currently.) When I boot that drive Sidux shows first, Windows
second, Ubuntu third and then the other distributions. Not all distros play
nicely. Fedora seems to ignore other distributions. When I install Fedora
then I need to cut and paste into Fedora's grub from other ones or boot into
a distribution from the grub on another drive and have it overwrite Fedora's
instance of grub.

You might think that grub seems complicated, but it isn't if you understand
the logic of it all and keep track of things. Grub is really very simple. It
is just a text file but it can (as in my case) do complex things.