Using a old Hard drive on a new PC Build Help

i been hearing from make youtubers to canabalise or get a hardrive from a old computer to a new build if you whant to save money the thing is that the harddrive that i have taken out has windows vista and alot of other files and a password that i forgot. What i dont know is that if the hard drive will keep all of this junk or will it all disapper when i took it out my old pc please help.

 

everything will still be on you just wont be able to boot from the hard drive at least in my experience if theres nothing on it that you want to save just format it and there wont be an issue.

When your going take take the hardrive out and put it in the new one it going to keep it data it not going to remove any window file or other files and password but i would still recomend backing up important data on a ubs but it will keep it old data it will even still have the old systeme gpu driver and such so yah from my experience i was able to boot from my old window which was in the hardrive it was weird the old cpu in it was a dual core when i shich machine the new machine was a 6 core but vista show 2 core anyway it will keep it data