  We present Quantum Monte-Carlo simulations of an exchange-anisotropic
spin-1/2 Heisenberg model on a square lattice with nearest and next-nearest
neighbor interactions. The ground state phase diagram shows two classical
magnetically ordered phases for dominating antiferromagnetic S^z-interactions
and for large quantum fluctuations a ferromagnetic order in the x-y plane. In
between a finite region is detected where neither classical nor quantum
mechanical order, e.g. long-ranged dimer correlations, are found.
