Are you sure you have installed PostgreSQL ??
If it's installed, you should see a service running (start menu, execute, type:
services.msc)
Check if you see a line with postgresql. If yes, is its status = started ?
If you don't have that line, you need to install postgresql
If it's not started, try right click + start this service
You've also got a FAQ here:
FAQ - Hold'em Manager Poker Tracking Software :: Can't Connect / Failed to Start PostgreSQL