When Flash Player is not working, have you checked to see if Flash Player is enabled?
Click Tools, click Manage add-ons, ensure that the Shockwave Flash Object is enabled.
When Flash Player is not working, what files are in C:\Windows\SysWOW64\Macromed\Flash?
I installed FP 11 and it did not work. I use Internet Explorer and Windows Vista. I went in to the "tools" pulldown menu and made sure the Active X Filtering was not checked. Flash Player started working after I restarted Internet Explorer.