Helper Application Does not Exist


I recently updated my Windows XP Pro x64 system with Endnote X2. Now when I try to download citations from places like JStor or ProQuest, I get an error message saying something like:

 <file>.RIS could not be opened, because the associated helper application does not exist. Change the assocation in your preferences.

I am using Firefox 3.0.1, and I did clear my cache by selecting Tools >Options > Advanced > Network > Offline Storage > Clear Now.

Does anyone know how to fix this?