"Shared storage" in a cluster has to be a SAN or iSCSI solution, the nodes need block-level access to the drives; a shared network drive does not qualify for this and can not be used for an Exchange cluster.
For testing purposes, you can use Virtual Server on a machine with enough RAM and HD space and implement a cluster on virtual machines.
Using Microsoft Virtual Server 2005 to Create and Configure a Two-Node Microsoft Windows Server 2003 Cluster
http://www.microsoft.com/technet/prodtechnol/virtualserver/deploy/cvs2005.mspxImplementing a Two Node Cluster with Windows 2003 Enterprise
http://www.msexchange.org/tutorials/Implementing-Two-Node-Cluster-Windows-2003-Enterprise.html