Sims 2 Extension packs not working



I bought Nightlife and University a couple of days ago. I have Sims 2 already installed and thought I’d finally get around to putting pets on as well whilst putting the other 2 extension packs on. All is fine until the end when I get a 'we will now check for updates, please insert CD 4 (of the sims 2) I do so and I get an error message and it all uninstalls meaning every time I try I have to go through the whole irritating process again. THe Cd isn’t scratched so why won’t it work. gra. Help?