If your current OS is OEM version then it is tied to your current motherboard. You could try to reuse it, but you might have to phone Microsoft up to ask them to reactivate it on your new motherboard. Which is not guaranteed to be successful.
If you have a retail version OS then you can just install it on your new build no problems.
The HDD is not tied to anything, so you can reuse your current HDD.
Ah Thankyou for your reply. I realize I could re-use the hard-drive, but I'd rather leave it complete with the build I have now, that way my wife can use this computer aswell. Nothing wrong with it, besides it's from 08. Not up to date and not up to speed
Thankyou all so much!! I greatly appreciate it. Everything would be compatible? Will be purchasing in around 2 weeks- I have enough now but want to make absolutely sure that I wont regret spending my hard earned money. At minimum wage and the cost of living, this can take an extremely long time to save up. Atleast having this computer will save me gas money from going to do things (Lol)