Whenever I have launchpad open in the background, I alt-tab and every seems to lock up. I can't seem to click on anything because they go into a "different tab" so to speak. The only way I found to fix it was opening task manager, even then I had to click twice to get it to open.
i think thats a windows issue you having, have you tried reinstalling launchpad, i get the feeling your on windows 7, that sometimes can be a little twitchy, mine is sometimes