iStorage Server is network storage virtualization software that can convert Windows servers into IP SANs. It allows expanding XenServer storage by using iSCSI to connect virtual machines to physical disks, partitions, or disk image files on Windows servers. The document provides step-by-step instructions on installing XenServer, configuring iStorage Server as an iSCSI target, adding the iSCSI storage to XenServer, creating a virtual machine connected to the iSCSI LUN, and installing an operating system on the virtual machine.