Given current complexity of Grid technologies, the lack of security of P2P systems and the rigidity of VPN technologies make sharing resources belonging to different institutions still technically difficult. We propose a new approach called "Instant Grid" (IG), which combines various Grid, P2P and VPN approaches, allowing simple deployment of applications over different administration domains. Three main requirements should be fulfilled to make Instant Grids realistic: 1) simple networking configuration (Firewall and NAT), 2) no degradation of resource security and 3) no need to re-implement existing distributed applications. In this paper, we present Private Virtual Cluster, a low-level middleware that meets these three requirements. To demonstrate its properties, we have connected with PVC a set of firewall-protected PCs and conducted experiments to evaluate the networking performance and the capability to execute unmodified MPI applications.