WorldCom, which carries almost half of the world's Internet traffic, is saddled with $40bn (£25.4bn) in debts and became the largest bankruptcy in US history after it filed for Chapter 11 bankruptcy protection following the discovery of a hole in its accounts.
Capellas, the chief executive of Compaq before it merged with HP last year, said, "I took this job because I am convinced that WorldCom has the assets, the customers and the people to regain a leadership role in this industry.
"In order to do this, we must first regain trust and win respect. Accordingly, together we will rebuild WorldCom into a model of good corporate governance and management integrity."
The US government this week said it would renew a major voice, data, wireless, video and Web hosting contract with the troubled telco.