Like it or not as a minor living under her roof with her as your legal guardian you have few rights to be honest. Technically as a child everything you own belongs to your parents, even if purchused with your own money. It could vary state by state in some cases, but it generally falls under the guidelines schools constantly use to enforce dress codes, locker searches, and similar things, kids (even teens) simply do not have the same rights as adults. On the other hand however, your parents can be held criminally liable for your actions in a lot of cases even if you acted totally alone.
I'm not a huge expert on civil law, and while there have been rare exceptions with kids sueing their parents in extraordinary circumstances, if this situation ever came up in a court I'd imagine you'd get spanked hard, assuming the court even agrees to hear the case and doesn't just toss it over to social services.
Understand, I'm not taking her side, just being pragmatic. I don't know enough about the situation to take sides.
If I had to go by my gut feelings however your probably shirking other things you need to do because of that computer. Taking it away from you, at least for a time, is called "parenting". It's just like if you manage to buy an "M" rated game and your parents decide to take it away from you despite argueing "hey I worked my summer job and bought it with my own money", I'd say they're doing exactly what they're supposed to.
Such are my thoughts.