I'm not sure if I can help or not. It might be your pop-up settings. Go to 'Tools' -> 'Options...' -> 'Content' and see if you have the pop-up blocker turned on. If you have it on, then you can click 'Exceptions...' and see if you have chess.com on that list. Maybe the migration changed your settings. It is just a thought.
Edit: I tried changing my settings to block pop-ups at chess.com to see if the tactics trainer and chess mentor windows were considered pop-ups. And they still worked with pop-up blocker on. I'm guessing the pop-up blocker only blocks the ones that try to come up automatically. This means that this might not help you at all.
Hello, I've just migrated to the new Win7 OS and for some reason, the tactics trainer and Chess Mentor don't work (I use Firefox). The training windows pop-ups don't appear when I click the <start training> button.
I have also installed .Net Framework version 1 - Is it possible there is any relation between the two ?