If you have Half-Life, Opposing Force, and/or Blue Shift, and you install them via steam, you are probably missing out on the High Definition Model Pack.
The High Definition Model Pack originally released with the Half-Life Blue Shift expansion. It dramatically improved some of the character models used in all the Half-Life 1 games:
It is not enabled in the Steam installations of the Half-Life 1 games.
From this FAQ, to enable, just have steam running and click this link.
If for some reason you wanted to disable it, use this link.