Currently Being ModeratedFeb 10, 2013 6:03 AM (in response to V.med)
Welcome to the Apple Support Communities
Try installing in safe mode. First, press the Shift key while your Mac is starting to start into safe mode, and then, open the OS X Mountain Lion installer (on Applications folder) and try to upgrade.
If it doesn't work, I recommend you to burn the Mountain Lion installer onto a DVD or USB drive with Lion Diskmaker > http://liondiskmaker.com Then, after creating it, press the Option (or Alt) key while your Mac is starting, select the OS X Mountain Lion drive and install OS X Mountain Lion
Currently Being ModeratedFeb 10, 2013 6:04 AM (in response to V.med)
If you are talking just about Flash Player, there is apparently* a bug in the installer. I don't think it affects all, and Adobe is working on an updated installer.
*Sorry, I don't have a link to the thread where a person stated they contacted Adobe and discovered this problem.