snipI just nearly posted the Source way of doing it. Time to start reading what section people post in.
Anyway, time for the Goldsource way:
You need to find C://...steamapps/<name>/half-life/resource/ClientScheme.res and open it with notepad. Then you can find a
few sections like, for example:
"MenuLarge"
{
"1"
{
"name" "Verdana"
"tall" "18"
"weight" "1000"
"antialias" "0"
}
}
NOTE: This example is the title text font.
And replace Verdana with whatever you want. TrueType font is recommended as I'm not sure with GoldSrc whether or not they need to be. Also not sure if GoldSrc needs to precache fonts.