Hi,
what user account does my .net wcf service run under when I am hosting it in iis 7?
Is it the IUSR_machine account?
What is the easiest way for me to run under a different account? (just for this service, don't want other things hosted in my IIS to be effected).
To change just the single application, you'll need to create a new application pool and change the identity for that pool. That way, every other application will continue to run as-is, as the applications that use your new app pool will use that new identity.
To set the account used by anonymous users for a particular site, open the properties for that web site. Then, go to the Directory Security tab. Click the "edit" button in the authentication and access control box. In the window that appears you can specify the user account which will be used for anonymous access to the site. This will control the account used to access files, but may not apply to ISAPI applications (in which case a new application pool will be needed with its own permissions as already mentioned).