Here is the full list of all 21 Plants vs. Zombies GOTY Edition achievements. It takes around 40-50 hours to unlock all of the achievements on Windows.
Defeat a normal roof level without using any catapult plants.
Get to 20 flags in Survival Endless
Defeat 2 Gargantuars with Corn Cob missiles in a single level.
Get 8000 sun during a single level.
Complete a daytime pool level without using pea shooters of any kind.
Complete a nighttime level without planting any Mushrooms.
Bowl over 5 zombies with a single Wall-Nut.
Get to a streak of 10 in I, Zombie Endless
Blow up 10 zombies with a single cherry bomb
Hypnotize the lead dancer zombie
Get to a streak of 15 in Vasebreaker Endless
Blow up a zombie using a potato mine
Pick up 30 coins in a row on a single level without letting any disappear.
Complete a daytime level by planting only Mushrooms and Coffee Beans.
Enable Mustache Mode
Grow the Tree of Wisdom to 100 feet
Beat all 20 mini games.
Get the golden sunflower trophy
Collect all 49 plants
Discover the top secret zombie
Complete adventure mode