I don't know the cause, but my W7 machine(clean installed in October 2010) is doing the same thing. I am getting random explorer.exe crashes and ntdll.dll crashes with no apparent reason.
I don't know for certain, but it seems likely it is being caused by a recent update. Have you updated any graphics card drivers lately? I just upgraded to nVidia's 270.61 and it seems like since then my system has gone downhill. Maybe just a coincidence but in my experience the computer world rarely has those.