1. Install the Flash plugin
2. Close IE
3. Run regedit
4. Go to HKEY_CLASSES_ROOT\CLSID\{D27CDB6E-AE6D-11cf-96B8-444553540000}\MiscStatus\1
5. Change the value from 131432 (or whatever) to 1024

This disables ALL Flash, and stops it from prompting you to download it.  Remember to save the original number in step 5 in case you ever want to turn it back on without reinstalling.

NOTE:  This will also disable any websites that use flash!  There are quite a few!  Flash is not just used for ads.