Can Someone Explain iTunes To Me?

Everyone tells me how wonderful and easy it is to use iTunes (OK, mostly Janak Parekh), but it must have been designed for people smarter than I am - I installed iTunes on a different computer and wanted to sync my iPod Touch with it. I had iTunes installed on my software testing computer, which has since been wiped out, so I need a permanent home for iTunes. Here's the problem I'm running into:

I've set it up to manually sync everything, because it seems if I don't do that, iTunes will wipe everything out on the iPod Touch. In iTunes, when I go FILE > TRANSFER PURCHASES, I get the above error message. You might be thinking "OK, your computer hasn't been authorized" - but it has. The screenshot below shows that.

So what's the deal? If I tell iTunes to auto-sync apps, it tells me it's going to wipe out all my apps on the device - but if I tell it to transfer the purchases from the iPod to the desktop computer, it tells me the computer isn't authorized. I only have one iTunes account - I've verified that the email address on my desktop computer matches the email address on my iPod Touch. Is there a way for me to re-download the apps I've paid for once they get nuked from my iPod Touch? Any ideas on why I'm seeig that error message about my computer not being authorized?

Side question: how do I de-authorize those other two authorizations that I don't have/aren't using?