Download the Flash uninstaller first of all.
Then download 10.3 from here: http://forums.adobe.com/thread/889580
Disable your AV scanner before running either. If you're using AVAST antivirus, right click the orange icon in systray, go to "Avast Shields Control" and choose to disable all shields for one hour.
If you use a different AV, consult this list for instructions on how to disable it: http://www.bleepingcomputer.com/forums/topic114351.html
Then run the uninstaller followed by the 10.3 installer.
Re-enable you AV after the installation completes.
You may need to disconnect from the Internet during the install, to prevent the installer from checking for newer versions.