- Install the Flash plugin
- Close IE
- Run regedit
- Go to HKEY_CLASSES_ROOT\CLSID\{D27CDB6E-AE6D-11cf-96B8-444553540000}\MiscStatus\1
- 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.