For a start, do you use Steam? (I have a feeling you do due to the "3rd party mod message"). Secondly, have you installed Half-Life 1 properly, and the Open Beta in the Half-Life folder? This game will not work with Half-Life 2 nor any source versions.
If that was the case, he'd be pretty lucky to find an esf server??
EDIT: wait, I think you're right... this happened to me about a month ago when I couldn't log into Steam for about a week.. I reinstalled WON CS retail and put ESF on that instead, then accidentaly clicked find servers.