I'd first suggest you check your task manager and close down Bridge, if it is running, before the PRE10 trial install.
Also right-click the setup file and select 'Run As Administrator'.
If that doesn't help check task manager and close down all other Adobe background processes.
Insanity is hereditary, you get it from your children
Thanks Neale for your help.
First I checked for Bridge in the task manager: no such process running.
Then I uninstalled PRE10 and next reinstalled it by selecting 'Run as Administrator'. This did not solve the problem: the error message reappeared.
Finally I checked the task manager for other Adobe background processes, but there was no such process.
It appears that after closing PRE10 an application called 'Adobe Media Core CS6' appears in the task manager, and next the error message appears saying that "Adobe Media Core CS6 has stopped working".
I really don't know if this actually is a problem, and I don't know if I need Adobe Media Core CS6 for anything. My applications in CS5.5 (Photoshop etc) and also PRE10 work fine. But I would like to know if I can ignore the error message before I buy PRE10.
Hopefully anyone can help me.