How can I update iPhoto or iMovie (which were preloaded on my Mac mini, OSX 10.8.2)?

The App Store tells me I have updates for iPhoto and iMovie. But when I click on them, it tells me I need to sign into the account with which I purchased them. I'm signed into the app store, but I never purchased these apps, since they came pre-loaded when I bought the mini (new) in August '12. So how can I update them? (No problem updating other things like the OS, btw). Here's a screen shot of what I always get:

This has been a problem since the beginning -- I've never been able to update them, and the update reminder permanently shows 2:

Thanks in advance.