I think I know whats wrong... you say that you are using Flight Simulator X with the 707 yes? Well, the 707 isn't FSX compatible. That's why. But Captain Sim is planing to make it FSX compatible. And Captain Sim, I don't think it is Windows Vista either. I have the 707 and Windows Vista and it runs perfectly.

Now, for your 727. I don't have this add-on but I'll try to help anyway. The only thing I can think of is that you didn't install Flight Simulator right. Try uninstalling and re-installing it. I think that should work. But, I don't know if the 727 is FSX compatible. Did you try using both airplanes in FS 2004? Oh, and try using the air crafts in FS 2004 without FSX installed on you computer.

Hope this helps!  ;)

(By the way, cool background on your desktop!)