Replacement computer subscription problem!

I purchased a new Dell computer a couple of weeks ago. I installed NIS on it using one of the activations I had left on a three machine version I purchased in February 2012. The machine had a motherboard problem and after trying a replacement motherboard, they shipped an exchange computer complete. The hard drive was swapped out of the original machine into the replacement. NIS was working fine for a few days. Today it reported that I had no activations left and prompted me to buy another key! HELP!