F12 not working

Strange problem with Flash in CS4 designer suite. F12 in flashCS4 publishes ok but suddenly doesn’t open IE8 in flashCS4. Worked fine untill now. In dreamweaver CS4 the F12 opens IE8 without any problem. Must be a file or registrykey corrupted but where to look.
Allready tried CCleaner, RegFix an a new installation of Flash. Nothing worked.

Use Vista + IE8