Thanks again H -
I've run the powercfg commands again -
lastwake reported "USB Composite Device" and that's my mouse/keyboard - as expected
-devicequery wake_armed reported the Logitech keyboard/mouse - again, as expected
the -energy report listed 14 "errors" but none of them appear related to preventing Standby.
NVIDIA released another driver
for my GTX 470 so I'll update that now...otherwise, I don't know what else I can do besides what you suggested, going back to a default Power Profile, deleting the Homegroup and starting over.
I've run the sfc utility and no problems were reported.
One question - do you enable S1 or S3 in your BIOS? I always heard S3 was unstable and that most users preferred using S1.
Thanks again for your help!