HELP ME PLEASE!????/Wondows validation product key?

okay, my bf had someone attempt to fix my computer which is a dell 3000 or something like that but it's about five years old or more. I am the only owner. his friend put some fake windows on my computer. I was told that all I need is my product key to validate it...HOW DO I DO THAT? This windows genuine validation ish is taking me to a bunch of dead ends. Ijust want to get to the point where I enter the freaking key and get my computer running again. Any help?