iTunes uses an "authorization" system for controlling DRM content you've purchased through the store. That mostly doesn't apply to music anymore, but it does to any video content, including the "digital copies" that accompany many DVD and Blu-Ray movie titles today. It limits you to authorize five computers per iTunes Store account. But there's three hiccups:
1) Authorization is bound to system configuration. On Windows, this means the combination of peripherals, both on the motherboard and in expansion slots. On Macs, it really just means the built-in ethernet interface. If you add, change, or remove PCI cards from a Windows box, or you change the motherboard/logic board from either a Windows or Mac system, you have to re-authorize the computer, and it's treated as a new auth--your old auth still exists in the iTunes Store's database, even if you can't use it anymore.
2) You can only deauthorize an individual computer FROM that computer, while the authorization is valid. If you forget to deauthorize before you change the system configuration--or before you send your Mac in for service, and they wind up switching out the logic board, you can't deauthorize. Again, you can only reauthorize anew. Also, if your hard drive crashes, or you erase it, your authorization token disappears with the drive--again, you have to reauthorize anew, consuming one of your five slots.
3) You can't deauthorize one computer from another.
4) If you use Boot Camp on an Intel Mac, and you authorize it both under Windows and OS X, that counts as two computers.
Over the nearly ten years that iTunes has existed, I've owned six and a half different Macs and a Windows box. I've had to send two of the laptops in to Apple for service, which entailed replacing the motherboard--with its accompanying Ethernet interface. I forgot to deauth them before I sent them in, so their authorizations went "poof". As of yesterday, I was at 4 authorizations. Once I auth'ed the new Mac Mini, I'm at 5... which unlocked something Apple quietly introduced four years ago. Use iTunes to log into the store, and bring up your account information. Once you have five systems authorized, a "Deauthorize All" button appears next to your auth count. You can use that button once per year to wipe off all your authorizations. And then you can reauthorize all the systems you still want to use, starting the count over at zero.
 A Hackintoshed Dell netbook isn't Apple hardware, of course.