Transferring apps from mulitple computers

Hi! We have multiple home computers and kids that have itouches. My husband just got one too & I can't figure out how to share apps. My sons got his itouch over 6 months ago & I don't want to have to buy apps again. My husband has been sync-ing his touch to my laptop & my son to one of our PC's. How can they share since we've purchased all these apps without losing any? Does this make sense? My laptop is XP7 and my PC is just XP.