It's possible the WMC WTV container requires more processing then the built-in aver software, which recommends a p4 2.4 with hyperthreading. Your CPU is missing hyperthreading, which could be the difference, even though it has 2 cores.
But my C2D e8400 only uses about 5% CPU when recording per tuner with an Aver A188, which should be the same tuner as yours, and I don't think the e8400 is that much better than the celeron E3300.
What process is maxing out the CPU when recording? What does the task manager/resource monitor say EHREC is using while recording?