Activation

I have reput Windows vista into my one computer 5 times I had to take it out the last time cause it locked up when i tried to activate it. I have this computer only with window vista ready to reinstall, if i can put it into this computer without having to buy a new product code. I feel I should have not cause to buy a new key when i only want it on this computer and have not tried to put it on any other computer. I have had nothing but problems of trying to put this on my computer and the last time i put it on i figured woot i finally did it right. Well wrong as soon as it said i had to activate i went to not being able to do anything but reformatting and putting windows xp back into my computer. I would love to put Vista back in but it will do me no good if after 28 days i end up not being able to activate this product. I have registered this product and do not feel I need to buy another product key. I spent 158.00 on this one. What do i need to do so i can reput this into my computer without any extra cost or pain of redoing every 3 days weather i want to or not?