I started to download the latest edition of adobe flash and the download got interupted leaving a white box in the middle of my screen in the same footprint as the adobe download window. I called apple support and went through all the restarts and eliminated all the installers and it is still there. It covers a large part of the middle of the screen and basically makes the iMac impossible to use. The apple tech said im probably going to need to have someone walk me thru completely removing and then reinstalling adobe flash. We cleared the cookies and there is no malware. The white box does not appear when you restart into safe mode.