Is my GPU assisting the video decode?

Ive just bought a new laptop for uni and it has a x4500mhd IGP and a P8400 CPU, whilst my main rig is uses a Radeon HD3850 512MB and a Q6600. Ive noticed that when I playback a HDDVD on my laptop the cpu usage goes up to something like 50-60% even at relatively low res (1280x800) but when I play the same disk on my main pc i get like 5-15% CPU usage at 1366×768. (Power DVD 7.3 Ultra with clearvideo/avivo enabled)

Clearly the HD3850 is helping with the decode but why is the CPU usage sooooo different? Even in the catalyst control center the GPU never goes over 3% during playback so its barely flexing its muscles.