Here is the full list of all 50 Call of Duty: Modern Warfare 2 achievements. It takes around 15-20 hours to unlock all of the achievements on Windows.
Kill 4 enemies in a row while downed in Special Ops.
Earn 69 stars in Special Ops.
Earn at least 1 star in each Special Op mission.
Earn all 3 stars in at least 15 different Special Op missions.
Collect 45 enemy intel items.
Collect 22 enemy intel items.
Kill 20 enemies in a row while driving a vehicle in Single Player or Special Ops.
Kill 10 enemies in a row using akimbo weapons in Single Player or Special Ops.
Earn at least 1 star in 11 different Special Op missions.
Earn all 3 stars in at least 10 different Special Op missions.
Earn 30 stars in Special Ops.
Earn all 3 stars in at least 5 different Special Op missions.
Earn 21 stars in Special Ops.
Kill a Juggernaut in Special Ops.
Plant the C4 in 'Cliffhanger' without alerting or injuring anyone in the blizzard.
Run The Pit in 'S.S.D.D.' and finish with a final time under 30 seconds.
Complete the single player campaign on Hardened or Veteran Difficulty.
Complete 'Just Like Old Times' and 'Endgame' on Veteran Difficulty.
Complete 'Loose Ends' and 'The Enemy of My Enemy' on Veteran Difficulty.
Complete 'Of Their Own Accord', 'Second Sun', and 'Whiskey Hotel' on Veteran Difficulty.
Complete 'Contingency' on Veteran Difficulty.
Earn 8 stars in Special Ops.
Complete 'The Only Easy Day... Was Yesterday' and 'The Gulag' on Veteran Difficulty.
Kill 4 enemies with 4 shots during a slow-mo breach in Single Player or Special Ops.
Kill 6 enemies in a row using a thermal weapon in Single Player or Special Ops.
Complete 'Wolverines!' and 'Exodus' on Veteran Difficulty.
Kill 2 rappelling enemies in a row before they land on their feet in Single Player or Special Ops.
Complete 'Takedown' and 'The Hornet's Nest' on Veteran Difficulty.
Kill 7 chickens in under 10 seconds in 'The Hornet's Nest'.
Complete 'Cliffhanger' on Veteran Difficulty.
Knife an enemy without him ever knowing you were there in Single Player or Special Ops.
Complete 'S.S.D.D.' and 'Team Player' on Veteran Difficulty.
Earn 4 stars in Special Ops.
Earn 1 star in Special Ops.
Complete the Single Player campaign on any difficulty.
Complete the mission in the airplane graveyard.
Assault Makarov's safe house.
Kill 5 enemies in a row using 5 different weapons or attachments in Single Player or Special Ops.
Take back Whiskey Hotel.
Execute the plan to help the Americans.
Storm the gulag.
Use a riot shield to beat down an enemy in Single Player or Special Ops.
Defend Burger Town.
Kill at least 10 enemies with one Predator missile in Single Player or Special Ops.
Find Rojas in the Favelas.
Kill at least 3 enemies with a single shot from a grenade launcher in Single Player or Special Ops.
Infiltrate the snowy mountain side base.
Get hand picked for Shepherd's elite squad.
Kill 2 enemies with a single bullet in Single Player or Special Ops.
Help train the local militia.