Over at Interesting Findings & Knowledge Sharing there is a look at a Permissions Issue with SQL Reporting Services on Vista.
Of course, you could switch to Windows 7 instead. I’m just sayin’.
Permissions Issue with SQL Reporting Services on Vista | Interesting Findings & Knowledge Sharing
Archived post from 2011. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.