There are a couple of solutions probably. First off, search pgfoundry for possibilities, look into clustering and replication.
A little more insight would make it easier to answer your question: Determine what you exactly want, what kind of uptime at what expense. How should it be made available (single ip, multiple ips(requires application changes)) Do you wan't a full cluster (load balancing, distributed transaction, service provisioning, failover), or will replication suffice and running a separate heartbeat checking script) to effectively fail over. Is downtime an option (a couple of minutes) How is the rest of the environment protected against downtime What version of Windows is it(Standard,Enterprise,Datacenter) for MSCS you require at least Enterprise What other services will be running on the servers with PostgreSQL How many servers will you be running in the solution Have you previously configured a similar application (SQL Server,Exchange,ISA,etc) Is moving to another OS an option How long should implementation take