It was the second time that I’ve come across this error message (on a different machine). The previous time I just used the alternate install CD that allows you to perform certain specialist installations. Since this time a had not a spare empty CD I looked for a workaround. So here is it:
Run the liveCD and prior to the installation open a terminal console (Applications>Accessories>Terminal). Open the validation.py file for edit:
sudo gedit /usr/lib/ubiquity/ubiquity/validation.py
Replace:
if not root:
result add(MOUNTPOINT_NOROOT)
with:
if not root:
pass
That’s all. You can now run the installer.
(source: http://www.ubuntuforums.org/showpost.php?p=1700787&postcount=29 )
antonis.log() 
G’day,
On my install of Ubuntu Eft (Desktop), validation.py was one level deeper, in /usr/lib/ubiquity/ubiquity/validation.py
James you are right. I’ve corrected it. Thanx:P
Right and wrong. You can also delete all partitions except your windows partitions, automate the partition maker and finish the process and it will continue installing.
Antonis’ solution is right but in the end I followed Jeremy’s way to install ubuntu 6.10
This does not work with me trying to install 10.04 LTS on older Compaq laptop.
Issue is occurring from 2005 or so, please figure it out already dear Ubuntu developers.
If it can stop me from installing there is a huge mass of users who weren’t able and are lost forever, hating Linux.