Hi :)
Installing any version of Windows overwrites the Mbr. There is no avoiding it. The trick is to re-fix it afterwards by reinstalling Grub2 to that drive and then Grub2 picks up all the different OSes including both Xp and Win7. Of course Win7 is already "the old one" and Win8 is apparently a pain. When you want to fix the Mbr again Grub2 allows you to just fix the Mbr without having to wipe your current configurations but you might want to back-up your customised menu just so that you can copy&paste bits from it if the new one is a bit wonky.
However the big question is why bother with Win7 at all now? Most people are moving to mobile devices and most mobile devices are Androids, iSomethings, Blackberrys and only a very few Windows ones. Also a lot of Windows software runs in Wine although it takes a bit of time to set-up some programs. Wine stands for "Wine Is Not an Emulator" because it uses a far more sophisticated approach that runs things much faster. In some cases faster than on Windows. There is also "PlayOnLinux" although that is just Wine with extra bits to try to make it easier. With Wine you can set it, on a per-program basis, to act like any of the various Windows OSes, stretching back to Win98 and i think even Win95. So one program would think it was happily running on Xp while another would be convinced it was on Win7.
Also many programs started life on unix-based platforms or have equivalents that are native to Gnu&Linux. So, you might be able to dodge Win7 just as i dodged Vista and Win7.
Regards from
Tom :)