Game Save Location Database WWW.GSLDB.COM

Plants Vs Zombies

Steam:

Windows XP
C:\Program Files\Steam\steamapps\common\plants vs zombies\userdata

Vista / Windows 7
C:\ProgramData\Steam\PlantsVsZombies\userdata


Popcap non-Steam:

Windows  XP:
C:\Program Files\PopCap Games\Plants vs. Zombies\userdata

Vista / Windows 7:
C:\ProgramData\PopCap Games\PlantsVsZombies\userdata

Source: Steam forums