umm with opposing force, do you also have the original half-life and are you also able to play the other official mods like counter-strike and day of defeat?
if not, then i doubt you'll be able to play ESF with opposing force..
it would require Steam to play now a days, WON as we speak is nearly dead (the HL -stand alone engine network)
Steam is trying to create an entire new kind of gaming network which is really great! but you can't run ESF with Opposing forces. As Fatal_Cobrax is saying, Buy Hl, if you already have it, download steam and register as a new user, press on browse games, then in the left bottom there should be "Register a product I already Own", Insert your Half life cd and then type in the cd-key, install ESF afterwards and there you go! You are now ready to go kick some ass!