You can't take a drive from one system and put it into another and expect it to work. All of the drivers are stored on the drive, and they won't work with different parts, so that is out.
You'd need to back up all of your data, format the drive, and reinstall windows on the new computer. Then you can restore your files.
As far as purchasing of windows goes, no, you shouldn't need to purchase another copy.
You just need to recover your key, and use that again when you install windows on the new PC.
Again, you can't transfer drives with operating systems and drivers already installed on them unless the two computers are IDENTICAL.