Hi, I was recently restarting my computer when i got a blue screen saying that my registry went corrupt, and i have to reinstall windows. While doing so, can use the same key that I use now, without using up the last key in my subscription? I plan on using it on my parents’ computer because they need an upgrade.
yes, your key is valid for the amount of computer / years you bought it for
info is found in FAQ section, search and you shall find 
https://support.avast.com/support/home
use search word video for how to do videos
so, if i remove my old version of windows from my avast account, will i get one space freed up for another computer, because i only bought 3 keys, and 2 are used up now
Since it’s the same computer, you shouldn’t have to remove anything. 
even with a completely different version of my operating system?
It’s still the same computer and the same hardware. I suppose it wouldn’t hurt to remove
the current computer from you my.avast.com account.
alright, thank you very much!