Ahh,so this model is from Natural Selection, good ole Half-Life mod...
I took a look at your max file, and I'm kind of puzzled on the way you rigged this. First the, the biped head is nowhere near the head on the model. Also the legs are kinda funky too, the toes are pointing straight up. I see you're using my modified zombie biped.
So I deleted the physique modifier to see the original model. It looks totally different. I would start over and rig this again. Also, I would add a tail link and ponytail link to the biped.
Here is what it SHOULD look like once its rigged properly. I'm not sure the default FPSC biped will work properly with this model. I think character studio has a "reverse knee" option, although I've never used it yet, it would be appropriate for this model. And it looks like you should add another leg link while you're at it.