Power off your Htc Eris, if phone is frozen remove the battery and place it back in the phone.
With the phone turned off, press and hold the Volume Down key, Send key (green phone) and Power key simultaneously.
The device will turn on. Keep holding down these keys untill you see a white screen with the words clear storage.
Press the track ball to confirm you want to clear the phone of all storage and data.
The phone will perform the factory reset, reset the phone, and bring you to the intial setup screen.