Here is the full list of all 20 Poker Night at the Inventory achievements. It takes around 20-25 hours to unlock all of the achievements on Windows.
Win a hand with Four of a Kind.
Win a hand with a Straight Flush.
Win a showdown with just a high card.
Win a game on hard.
Knock out Strong Bad after he's bought in with his special item.
Knock out The Heavy after he's bought in with his special item.
Knock out Max after he's bought in with his special item.
Knock out Tycho after he's bought in with his special item.
Win consecutive all-ins.
Win a pot with two or more opponents when you are all in.
Check a pat hand.
Win a hand with a Full House.
Win a hand with a Straight.
Win three straight pots.
Win a hand with a Flush.
Win a full game.
Win 12 or more hands.
Win a hand when you have three of a kind comprised of three kings (of any suit).
Win a showdown where you and the opponent have same hand (pair or better) and you have a better kicker.
Win a hand with a 6 card and a 9 card of any suit.