Never mind. Just solved it, OS X, Ubuntu, and XP professional work fine together now.

I will record what I did, since maybe this will help someone having this problem in the future.

As stated above, I used FIXMBR from windows XP CD and this did not solve the problem.

I then tried to synchronise the partitions through rEFIt partition tool. That reported that all Boot tables were synchronised and that there was no need to do anything.

However, the rEFIt partition tool, showed me that windows is installed in partition 4.

I then checked the windows boot.ini to see what partition it is trying to load from. I found that boot.ini is trying to boot from partition 3 instead of 4. So, I changed the two lines that are referring to partition 3 to point at partition 4 instead.

This fixed everything. All work fine now. I can boot from all of them.

Sorry for the false alarm.