I've been using VC on my laptop for a while now, usually using the built-in mic. For ordinary uses (like Skype) I've usually had to turn up my input levels to get people on the other end to hear me, but it's clear now that VC has different needs. It was picking up everything, and I was getting a lot of false positives.
James suggested that I try testing the levels by turning the input *way* down. I turned it down from 70 (already a lower input than I used to need) to 10 -- this has resulted in a *major* improvement in the accuracy level. So far, it's working perfectly -- understanding what it's supposed to and ignoring random utterances or background noise. (Granted, I live in a quiet home.)
If you're experiencing issues like this, I highly recommend trying this: start low, and increase if that doesn't work, rather than the other way around.