dont be, because it's actually not related to the OS. The boot loader Ubuntu uses is called GRUB, it's been around for ages and loads just about every OS known to man kind. Point being that since the first half of grub resides in the MBR, it's not the part that loads your OSes. Ubuntu runs a script that detects Windows OSes and creates entries in the config file to load them. You can always back up your current grub.conf (located in /boot/grub/grub.conf). Just make sure that after the 8.10 install you take the windows parts out of the old config file and don't change the kernel load for the 8.10 kernel.
your windows parts should look like this:
(this is assuming that windows XP and Windows Vista are installed on hardrive 1 partition 1 and 2 respectively, the hd(x,y) part changes)
Code:
title Windows XP
rootnoverify hd(0,1)
makeactive
chainloader +1
boot
title Windows Vista
rootnoverify hd(0,2)
makeactive
chainloader +1
boot
but this actually invokes the windows boot loader on that specific partition so you can overwrite without worrying about the boot loader, in reality you could force ubuntu to not touch the boot loader, but its too much worry for your situation, as far as i can see, well, read
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Microsoft, the leader in using innovative tactics to promote irksome experience, coupled with antiquated technology that's held together by a pyramid of makeshift afterthoughts.
Apple, the leader in using irksome tactics to promote innovative experience, coupled with an antiquated core that's enhanced by state-of-the-art afterthoughts.
Linux, the leader in not using any tactics to promote user-defined experience, coupled with state-of-the-art core enhanced by innovative afterthoughts.
