Key code

I know this question has been asked.every which way but sideways, but here we go again. I had to put in a new hard drive because of the no bootable disk error. So I did. Then i had to buy a boot repair disk and I was able to download some drivers to get it going. My question starts with the keycode for windows. My windows is working i can go to the internet and installed word etc but it still wants a new code, says the present one is invalid, even though it was the one that came preinstalled. I contacted Microsoft and they said it will slowly stop working. Anyone ever heard of this. They suggested I buy windows 10 or... They said to contact acer for a keycode, but my warranty is over and I get nowhere on their site. Will it indeed slowly shut down on me?