Steven Walters Law Firm review: harrasment

I have been recieving 'threatening' phone calls from a Michelle claiming to be a lawyer from Steven Walter Law Firm.
She is demanding a payment for a computer that I PAID OFF back in July that I got through MDG computers. Apparently I owe a $300 "buy out " fee.
I have ZERO intentions of making this payment as I specifically asked MDG when my last payment was due, which I was told July 2010 and when I asked "so I owe you no more money" I was told no.
My question is, if I still owed this $300 why did I hear nothing from MDG for 6 months either by phone or mail?
As far as I am concerned this is a scam and I WILL NOT PAY one cent more for something that isn`t even working at the moment!
Take me to court I dont care u can`t get blood out of a stone so they say.