The licensing terms of Snow Leopard unfortunately don't allow that to happen, and Parallels unfortunately also, enforces these licensing terms by detecting and blocking the installation of SL. However, Apple allows SL Server, in its license, to be virtualized and so Parallels permits its installation. So, if you have a copy of SL Server you can use that.
To the frustration of many of us this is not a technical limitation but an artificial licensing limitation only.
