It would be done with the videocard software. Enable FSAA before playing HalfLife, then it'll work.
I don't have a Geforce, so I can't give specific instructions, but somewhere in the Video Properties there is probably a way to set individual settings such as FSAA for individual games. That's the best way to do it.
If there isn't, then you'll need to try some kind of Geforce game launcher, which you can find on the net.