how to I check if the service is running and where can I find the logfile?
In the Control Panel there is the "Services" applet.
The logfile is usually in the data directory that you specified during installation.
My bet is that you either did not initialized the database at all or that the permissions on the data directory are wrong. Search this forum for problems with file permissions under Windows. There are plenty of questions around this.
If Postgres should have been installed during the installation of OpenERP it's probably better to ask in an OpenERP forum regarding installation issues.