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Ubuntu automatically take care of GRUB and the MBR?
hmm... as far as I know, there is no problem with installing over another distro. Recently, I installed FC3 over RH9 on a system having winXP as well in dual-boot.
I selected the same partition as RH9 was...and everything worked perfect, with no problems. Even WinXP was bootable using GRUB.
But while installing Ubuntu, remember to install the bootloader.
[Caution: I have never worked with Ubuntu, so no ideas about the distro. But if this is a generic question that you have asked, answer would be :No problems with installing a distro OVER another, and still keeping intact other OS..]