A desktop cloud is an implementation of hosted virtual desktops using cloud computing technology. Desktop clouds are one of the most popular applications for cloud computing. In this work, mechanisms for secure accessing and sharing of virtual desktops are investigated in details. The public key infrastructure (PKI) is utilized to create virtual organizations (VO). Within a VO, virtual machines are created, remote desktops are accessed and shared. PKI provides security mechanism and multiple users can share a virtual machine via VO trust management. In order to make remote channels secure, OpenVPN is adopted to build a private network, authenticating users and encrypting communications.