Long time ago I had to rebuild a cousins computer because it was crashing from all the malware. After I finished cleaning it, I plugged it into the cable modem

Long time ago I had to rebuild a cousins computer because it was crashing from all the malware. After I finished cleaning it, I plugged it into the cable modem within seconds the computer was having pop-up ads, and every kind of malware you could imagine. So rebuilt it once again, then installed antivirus from a disk before hooking it up to the internet. It was actually functional.