SOS=black screen and unvalid key product

My computer had a trojan could not turn it on the computer i call (technical) and I fix it but when the settlement told me he was going to erase everything and subtract whole and that the computer had Windows XP Home edtion and change me for profecional said it was better but I spent a month on my screen it came out that I had to reactivate the window xp profecional because it was false or the product key was wrong to try to call the man but was never answered Inmagine at this moment my screen This leaves me black and only turn it on if it is to put the new product key entered the internet to microsoft activation assistance website but does not give me anything I can not open a file try using the boot mode but not work or did not do well please someone help me please