Unable to upgrade to PD4 ?

Hello,
I've been using successfully PD3 for several months now, XP SP3 on a BootCamp partition with 31Go (Fat 32).
I have noticed though, that since several weeks, I cannot boot directly on BootCamp anymore (with the "alt" cap), XP will ever get me back to the blue screen, but it will still boot from PD3 (?)
Nevertheless, I tried to upgrade to PD4. The upgrading of the virtual machine went on for a third of the process and then stopped, asking me to go on "manually" (!) after XP would start. But then XP does not start any more...

Do you have a suggestion to make? Is there a way to save the machine instead of going all the way re-installing everything?