I have all my data on a separate partition so I would go through any files on the C Drive that are not programmes and delete them (Some keep e mail there - mine is on my data drive). Then copy all data to one or more external drives. Then delete all data, use CCleaner drive wiper on 3 passes - 5 takes many hours. That way the PC may be of use to another. I have in the past, used a hacksaw on a HD but it seems a pity.
You say disposing of it. It all depends upon whether you are scrapping it or passing it to a new user. a question that you have not answered. If you intend to scrap it then the answer is simple, remove the HDD.