On my desktop:
Upgrade from 9.10 to 10.04 hosed both ubuntu and xp - couldn't boot anything and tried LOTS of things.
Clean install using 'side by side' got me to grub rescue
Clean install on free space got me to flashing cursor
Never had so much trouble before
On the plus side, on my netbook:
flawless clean install next to Win7 and both working perfectly :-S