Yes, you can make a computer completely safe from hacking. However you probably would not view it as a usable computer today.

To make your computer completely safe from hacking will require a couple of things.

First item is to disconnect it from the network and Internet. You will need to make sure two things are done. First if there is a network cable connected, disconnect it. Secondly you will need to remove or disable the wireless card/interface if there is one in the machine. That will stop hacking from the Internet and network.

Secondly you cannot use a wireless keyboard on the computer as you are sending wireless signals someone can pickup and have hacked your computer. A wireless mouse is ok as the only thing transmitted from it is mouse movements and not knowing what is on screen they are worthless to hacker

Now that you have done this your machine will be safe, however you basically have a PC from the 1980s or early 90s except it runs much faster and has a lot more storage and memory than back then. Most of what we do on our computers today is using things on our network or the Internet so the usability of your machine is greatly reduced.

You can realistically do things to decrease the chances of being hacked. First make sure you have a current antivirus and antispyware program on your machine and keep them updated and run scans regularly. Secondly make sure your firewall is turned on in your machine. Windows includes a good basic software firewall and it defaults to being on. You can also buy other software and hardware firewalls. Third be a smart Internet user. Don’t click on links on web pages or in emails when they look suspicious. If you are on a PC hover over the link first and see if it looks like what it should look like and if it is an email is it from someone you know and does the message sound like something from them. Many people were burned by the I Love You virus in 2000 because they clicked on a link in an email from someone that they did not know or would not have sent message saying I Love You. If suspicious then don’t click it.