In order for Flash Player to be installed or updated, any programs that use Flash Player must be closed. This includes Internet Explorer or other browsers, instant messengers, Skype, etc.
These programs may be running in the background, not as visible windows, but need to be closed anyway.
To close such programs use the Windows Task Manager. To start Task Manager, take any of the following actions:
Once the Task Manager is running, select the Processes tab. On older Windows version you may need to click on the Process Name or Image Name header to sort the process names.
Now right-click on every process that the Flash Player installer needs to close (e.g. iexplore.exe), then select End Process.
It would be desirable if the Flash Player installer could perform that action by itself; I have therefore created an enhancement request. If you were affected by this problem, please head over to the Bugbase and vote for this request: https://bugbase.adobe.com/index.cfm?event=bug&id=3548136