Laptop not working properly!

I upgraded on 28 April 2015, and took an HP laptop at an MTN shop in Northgate mall. The laptop was not performing well, it was extremely slow, even opening a simple word document took minutes on end. I took it back to the shop on 5 May 2015. I waited for weeks whilst succumbing to appalling service from the shop, as they plainly refused to give me a turnaround time. After calling them countless times and eventually visiting them personally at the shop 25 May 2015, Kgomotso promised to call me either on 27 or 28 May 2015 to give me feedback, and of course as has been the case, she never bothered! I called 01 June 2015 and was told the laptop was delivered on 29 May 2015 already, and they never bothered to let me know! I fetched it on the same day, but the laptop is still slow and now it freezes when I use it! MTN I am utterly disgusted at the service I am getting from you and why am I sold a defective product when you expect payment from me month end? This laptop never worked well ever since I bought it on the first day. I demand a proper laptop else we can CANCEL the contract once and for all, this is unacceptable, I will NOT pay for a defective product!. SORT THIS OUT PLEASE.