If you think you can't get the files back, you sure as hell can get the machine back. Just go into her office and take that motherforgeter back into your space. You don't need to make a scene, or get revenge, or notify your parents (you really don't need to notify your parents). Just take it back. If she takes it again, take it back again. This back and forth exchange can only go on for so long, eventually she will confront you and then she'll have to answer for what she did in the first place.
Further advice, be EXTRA VIGILANT the day she goes back to college. If you're not home when she's leaving she might try to take it with her, in which case there's nothing you can do. Make sure she isn't able to try that. Protect what's yours and you'll be fine, it's not like she's going to hit you or anything. If she tries to physically snatch it off of you, she'll lose support from your parents in a heartbeat. That being said, don't provoke her either. If she never tires anything physical but you get your computer back, you got the best case scenario.
And again, you really don't need to notify your parents. They might be deliberately looking the other way in some vague attempt to teach you to stand up for yourself, so don't even ask them for help because they'll do jack.
Oh, uh, and tell her I said forget you.
