unrecoverable error on upgrading to professional

Hello,
I uninstalled basic version, installed professional version on my Win XP box.

Started client, upgraded to latest stable version, tested running 1 scrape session (which worked well in basic version 2.0). [It failed in professional version, receiving a null response, but that is another issue I might be able to resolve.]

Tried another scrape session of mine (also had worked well in basic version 2.0). This time the application froze up. I stopped it using task manager, and tried to restart screen-scraper professional client but get
"Another instance of screen-scraper is already running, either in server or workbench mode. The application will now exit."

I never started the screen-scraper service and it is not running. I rebooted my machine and get the same message when I try to start screen-scraper professional. I cannot start it.

Thanks for help,
Dado