Yes but you shouldn't replace anything. You would be better off reformatting the hard drive rather than replacing it. Better yet, try to fix the virus. Try combofix and malwarebytes antimalware. You could also try tdsskiller to check for rootkits. Replacing the RAM in any scenario will not have any effect, unless the RAM is bad. RAM doesn't go bad because of viruses. Replacing the hard drive and reformatting will yield the same results except reformatting is cheaper and faster.
Never plug the computer into the network and never attach its hard drive to another computer.
Yep. Just do a clean install and use the same Product code that is on the machine. Be sure to back up any important files so you don't lose them.