This is a basic and quite popular scam that uses social engineering to make gullible people believe there's something wrong with their computer in order to sell them some fake security solution and/or possibly steal data from the machine in the process.

The scammers are often based in India and use VoIP providers to call at cheap rates and appear as a local number, and for card processing they obviously use shady payment gateways that don't mind this illegal activity.

Most of the social engineering works by making the user open the event viewer and telling him that the errors in there (which are often benign and to be expected on such a complex OS like Windows with so much services running) are critical and mean that remote attackers are trying to take over the computer. They will then get the user to download a Remote Desktop server like TeamViewer (but TV has a warning specifically against these social engineering attacks so they fall back to some other, less known remote desktop solutions that have no such warnings) to take control of the computer and further show that the machine is in bad shape, sometimes by opening a command prompt and copy/pasting some command that produces lots of output with something scary appended at the end like the "tree" command with the text Warning: computer infected. Attacker's IP: 192.168.1.1 after it, which causes the window to list every single file of the system and display the text at the end as if it was part of the command's output, which unfortunately is an effective scare tactic. Once the user is convinced they make him go to their webpage to buy a solution which is often a fake security product, and assuming that's done I suppose they either just leave or install a rogue (fake antivirus). Note that most remote desktop services include a way to silently transfer files that can be used to exfiltrate confidential data from the victim's computer in addition to exfiltrating their money.

There are a lot of these scam calls posted on YouTube, you can look up "tech support scam" on there if you want. You can also play with them by setting up a virtual machine; just try to make it a bit fun (like creating actual malware, disguising it as a "passwords.txt.exe" with a text document icon and hope the idiots steal the file and run it on their computer).