PokerTracker

I have XP and am just beginning moving from PT2 to PT3.

I have no idea what Postgres is all about, and the first thing I am asked when I run PT3 for the first time is 'Server', 'Database User', 'Port' etc.

Is then a noob-guide or something or is it actually very simple?

If you run the PT3 'Full' installer then it will set postgres up for you. Postgres is the database server that PT3 uses to store its information.
Once you've done that it should just start up fine, but if it asks you for information the user is 'postgres' and the password is 'dbpass' (no quotes).