@CyberPunkNews: The implementation of Remote Desktop in XP (and later client versions) deliberately locks out other users to ensure that only a single user is using that machine at any one time, to ensure license compliance. Multiple users accessing the same machine via RD requires Remote Desktop Services on Windows Server (formally known as Terminal Server).
The fact that the ovf file is on one machine is irrelevant, that machine is simply acting as a file server. The Virtual Machine is the running instance of Virtual Box. Since each PC is running a virtual machine, each is effectively running a separate instance of Windows and each requires a Windows license. It's the same as if you bought a single user license of Photoshop and installed it on a network drive, you wouldn't expect everyone in the company to be able to use it by just running it from the shared drive (or at least I hope that would be obvious)
The Microsoft/Novell deal has done a great many things to improve interoperability. That does not extend to allowing Linux users free usage of Windows, for rather obvious reasons.