how can I drill a 1/2" diameter, 1/2" deep hole in 3/4" particle board?
Forstner bits (as mentioned by @keshlam) have much less of a protruding point, and would do an adequate job of drilling a 1/2" hole without poking though the remaining 1/4"

If you pick them up cheaply (bucket-o-bits at the flea market or yard sale) you can also grind off most of the point on a spade bit - start the hole with the normal "long-point" version, then change to one with the point severely shortened.

In the event that you want a truly flat-bottomed hole (which does not seem essential here), and if you happen to have the tools (buying them for the sake of making holes for cabinet legs is not economically sensible) a 1/2" plunge (or "end-cutting") router bit and and a plunge router will get you a totally flat-bottomed 1/2" hole 1/2" deep (if you set the plunge depth to 1/2", and hold or clamp the thing firmly in place so you son't cut a slot.)