This means that postgres isn't running.
Please try starting the postgres service: Start -> Programs -> PostgreSQL -> Start Service.
If that, and a reboot doesn't work, try stopping the service (Start -> Programs -> Postgres -> Stop Service), running a search on your computer for postmaster.pid and then deleting that file, then restarting the service and see if that helps.
Note that you may need to turn on 'View hidden files and folders', and the file seems to be in the folder './Program Files/postgresql/[version]/data'.
See also:
viewtopic.php?f=17&t=6332#p38285If that doesn't help, select the last few files in C:\Program Files\PostgreSQL\8.3\data\pg_log (they're dated so just pick the recent ones), right-click and choose 'Send To Compressed/zipped Folder', then attach the zip file to a ticket on the
Support system, and link to this thread in the reference field of the ticket so that we can investigate.