We have received a few reports of iScreensaver 4 misbehaving on certain systems running Mac OS X 10.6 or 10.7. In several of these cases, we have determined that a third-party system extension is to blame. Please note that although we strive for maximal compatibility, third-party software which hacks into or modifies the OS can cause severe problems.
So far, we’ve identified the extension “RightZoom” http://www.macupdate.com/app/mac/30591/right-zoom as one offending program. The solution is to remove RightZoom from your system if you plan to use iScreensaver.