Error starting pgAgent service on Windows 2008 R2 server

We've previously installed pgAgent successfully on Windows 2008 R2 servers but this latest server is doing our head in!  When we try to start the service we get an error:

"Windows would not start the PostgreSQL Scheduling Agent - pgAgent service on Local Computer.  Error 1053: The service did not respond to the start or control request in a timely fashion."

The PostgreSQL server is a 64 bit version: Version string PostgreSQL 9.1.10, compiled by Visual C++ build 1500, 64-bit.  We installed pgAgent using Application Stackbuilder.  

We've never seen this before and are stumped at this point.  Can anyone point us at something to look at?