Compaq (HP) has no such model listed on their website.
When you use the HP Quickfinder (link above) with "evo d51s", it returns results for the following models:
» Compaq Evo D300s CMT
» Compaq Evo D300s DT
» Compaq Evo D300s SFF
Edit: If you're referring to a EVO D510 model, see this page: