AvP 1 is on steam for $4.99. It doesn't include multi, but apparently that's being worked on and should be a free download when it's finished.

The difference here is that AvP Gold used to be a win98 only game. It wouldn't even work on XP. To use it on XP, Vista or 7, you'd have to use a modified executable that took the community years to get around to developing. Now it just runs.

The graphics definitely show their age, but it's AvP. 'Nuff said. Nothing can ever replace the beep of the motion sensor.