Hello I just installed after effects CS5 trial on my Mac. Although it will unfortunatly not load. The installation process was fine, and I used adobe cleaner to remove my old CS4 after effects so there would be no issues.
When I go to open it I am gretted with an error message saying it can not rewtie/rename or create the file Workspaces.xml
What Mylenium suggested should work. But, if it doesn't you can also safely delete the entire preferences folder (Preferences/AdobeAfter Effects/10.0). When you restart After Effects, it will rebuild the preferences files to their default state.