Purcahsed a new Laptop - whcih was Defective

I purchased a laptop from the MTN store in Gateway, I used same for month and realised that the Laptop was not functioning at its optimum. I then took it back to the store and it was booked in for repairs. I got the Laptop back four weeks later and functioned normally for two days thereafter it refused to start up at all. I took it back to MTN and was advised that a start up programme was deleted off the laptop. I then decided to send the laptop back to MTN for repairs, after a month and half after numerous visits to the MTN store I was advised that the \motherboard"of the laptop needed to be replaced. I was very surprised as the laptop was not even 6 months old. I then asked MTN to keep the laptop and give me a new laptop as the contract entered into between MTN and myself was for the purchase of a new laptop not a defective one. however I was advised that MTN could not authorise a new unit and was not given a reason for same. I was told to contact Adil Khan. I left numerous messages for him to no avail. I am extremely disappointed as I am paying my instalment for a defective product. I want a response from MTN!"""