Most often, these calls are to users at home and typically the person calling says they are from Microsoft and that malware has been discovered on the user's computer. As a "courtesy" they are just calling to remote in and help out - sometimes for a modest fee. What winds up happening is malicious software gets installed on the unsuspecting user's machine, personal information is compromised, and files are deleted.
Scammers do their best to look legitimate, often using a mix of spoofed phone numbers and social engineering in attempt to gain access or information.
From the official Microsoft Site:
Q: "Will Microsoft Ever Call Me?"
A: There are some cases where Microsoft will work with your Internet service provider and call you to fix a malware-infected computer—such as during the recent cleanup effort begun in our botnet takedown actions. These calls will be made by someone with whom you can verify you already are a customer. You will never receive a legitimate call from Microsoft or our partners to charge you for computer fixes."
For more information on how to avoid fake tech support calls and other safety and security information from our partners at Microsoft, follow this link to their Safety and Security Center.