If I understand correctly joao,
what will probably have happened, is that when you installed the SuSE you have accepted the defaults for bootloaders - which is fine.
It's written the bootloader (whichever either lilo or grub) to the first section of the MBR - main boot record - overwriting the windows one. Again, that's not a problem either.
You probably don't need to bother reformatting the partition that the SuSE's in (afterall, it's linux, not windows), just install the Ubuntu over the top, and accept the defaults or if it asks, tell it to install the bootloader (Ubuntu is debian/gnome based so I suspect grub) to install to the first section of the MBR (which is pretty normal).
then when you boot the system it would normally offer you the boot options window.
Erm the only thing that may annoy you is that lots of distros automatically make the linux install the default OS so if you just boot the system and don't choose anything it would automatically boot into whatever linux distro you have.
That can be changed so it's not really a problem.
Hope that helps a little?
p.s. If you wanted to re-install the windows boot loader, as far as I know, you usually have to use a windows disc or recovery disc if you got one with the system and "repair" the bootloader