I have a similar problem.. Normally, H&H runs fine on my laptop. (dual core 64 bit @1.6Ghz, 1G RAM, XP pro SP3, ATI radeon express 1100 with up-to-date drivers)
However, when I watch some streaming video's, and after that start up H&H, it will eat up 100% CPU, and be very, very laggy.
I'd say it skips drawing 19/20 frames.
I don't know if streaming videos (on megavideo.com) involves OpenGL. If so, it is a primary suspect for this behaviour.
Only solution I've found, is rebooting my PC.