You would NOT knock prone from the Assault Boots when throwing a melee weapon
Hungry Spear adds the following property:
This weapon has the heavy thrown property and a range of 10/20.
The DDI Glossary entry for both Heavy Thrown and Light Thrown weapons defines them as:
A thrown weapon is a ranged weapon that is hurled from the hand, rather than used to launch a projectile.
Items may have more than one type: they can be both "ranged weapons" and "melee weapons" and "implements" all at the same time. The type they assume at any given moment is based on the action being taken with them: when you take an action that has a "ranged weapon" requirement, the weapon that can be a ranged weapon is a ranged weapon for the duration of that action.
Likewise, you couldn't use Assault Boots with an implement power, because the Weapons as Implements entry specifies:
However, some magic weapons have properties and powers that work only with weapon powers.
If the power you are using is an Implement power, then the "weapon" is considered an "implement" for the duration of the power, and nothing that triggers off weapon, melee, or ranged hits would count.