How do I tell my mother that I want to buy myself a Mac?

I am 19 and I got a PC for HS graduation from my parents. This computer has Vista OS and is not really working for me. Despite my 2 antiviruses the viruses are still kicking my butt. I have a Dell whose hard drive has crashed twice, only now I don’t have the warranty that I had the other times. I have really done my homework and know that I want a MacBook. My biggest fear about just coming out and telling her that I am buying a Mac is that she will think I don’t appreciate what she got me. Please help me out!