Cannot install Flash Player 11.4 (following version11.3 uninstall)

Hello,

I get the following message when I run Flash Player 11.4 installer :

My English translation of this message is : "Only a single instance of this application can run"

Please take note that prior to running Flash Player 11.4 installer, I  uninstalled version 11.3 - using the uninstaller provided on your website - and then rebooted my PC.

After I got the above message, I took a look at the Windows task manager and found that two identical processes related to FlashPlayer installer were running!  These processes remained on the list for several seconds after I got the message.

How can this be possible, and most importantly, how do I resolve this issue?

Thank you

p.s.:  Here is some info about my system:

Windows 7 professionnal 64 bit SP1

Browser: Firefox 15.0