Welcome to Justanswer.com, my name is Dustin.I'm sorry to hear about the issue you are having, I will do my best to help you resolve it.
My first concern, is helping you contact your bank or credit card and disputing those charges, from the beginning this was a scam email and the bank should definitely take your side. Have you called your bank yet to dispute the charges? And also, you may want to consider having them issue a new card, with different numbers. Since that scam company has your credit card or bank card number you dont want them being able to charge you going down the road.
Im sorry to hear they got you for 3000.00$, but thankfully things like this are common enough that your bank should side with you. But you need to immediately dispute the charges and tell them you were victim to a a Microsoft support cold call scam.
Now, as far as the computer itself. I may be able to help you fix it. Lets start simple.
Whats the name brand and model of the computer?
And what version of windows is it running, windows xp, vista, windows 7, windows 8 or windows 10 ?