Windows 7 Boot Loop

Ok so i originally installed a windows seven upgrade from vista a few months back and my hard drive was getting pretty clogged so i reinstalled windows seven but i also hooked up a secondary hard drive so i could ave more memory, and originally the reinstall worked but now everytime i boot up my computer it says its going from first use and asks me to name the pc and put the cd key in, and i put the one i put in originally but now it says invalid

Check your boot sequence in BIOS