I don't know if this is really the place, but I really need

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I don't know if this is really the place, but I really need advice. I am currently 26 years old. Two years ago, my parents bought me a computer and paid for a proprietary firewall license and Microsoft Office Suite. I immediately installed Debian. I have had it on there for 2 years, with no problems. Yesterday, my step-father used my computer and I was forced to explain. They are now furious at me. I feel like a lot of it is their own misconceptions about Linux and how it works. They have expressed that they are confident my computer is now infected with viruses because I am not using the proprietary firewall they paid for. They are also worried that hackers will use my computer to get to communicate with their desktop and steal their banking information. They also want me to pay back the full price of my laptop, because I "pissed all over an expensive gift." This is all despite the fact that my computer continues to operate perfectly, with free software. I have tried arguing my case, but they keep asserting, incorrectly, that I am not an expert in all things Operating System and so they will not listen to me.
Any advice you can provide would be really appreciated. I don't really know what to do.