Paralles does not start on MAC OS 10.5

[Romano]

I have installed paralles desktop on my MacBook Pro, 2,6 GHz, brand new.
I installed Windows XP, service pack two, then parallels tools.
After 2 days working with the VM, I can not launch the program again, it always tells me that it did not start and asks me to either relaunch or to reset and relaunch. I have uninstalled parallesls since then and reinstalled, but there is no way to restart the VM again. Any idea what causes this ?
Thanks, Romano