From the AVD documentation:
By default, the android tool creates the AVD directory inside
~/.android/avd/(on Linux/Mac),C:\Documents and Settings\<user>\.android\on Windows XP, andC:\Users\<user>\.android\on Windows 7 and Vista.
Update (2020-02-22): This wording isn’t on the current documentation page anymore, but the location appears to be the same (C:\Users\<user>\.android\) on Windows 8 and 10.