I can't remember the last time I built a machine with only one OS on the physical hardware. You should have no issues putting a reinstall of Ubuntu on the hardware; maybe an upgrade to 8?
For me, the key is watching the partitions. Your drives are in letter order (hda = 1st drive, hdb = 2nd) and your partitions numeric order (hda1 = 1st primary, hda2 = 2nd primary). The hickup being the secondary partition wrapper which I think get's hda4 (partition 4) since hda5 is my first non-primary partition space. Numeric orders are left to write in the MS disk manager.
You may also try a Mandriva One liveCD at least as far as the disk manager (custom option) which will give you a good view of your partitions. From there, you can install Mandriva or note your paritions in an easily viewable manner then boot whatever you prefer to isntall.
Provided you don't forget which partitions bellong to Windows and format them, there should be no issue.
Bootloader is the other bit. If your setup to install your lilo/grub on a /dev/fd0 (floppy) then you can use that same setup and leave your hard drive FAT and Windows boot sectors untouched. If you have to install lilo/grub on the hard drive you may need to use your trusty Windows install CD and do an FDISK /MBR but be sure you've adjusted your lilo/grub first or plan to learn "loadlin".
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