Quote from PETE
"CMOS holds details of attached hardware while BIOS stores various settings and values that apply to that hardware as well as motherboard settings. If the hardware has changed for any reason, CMOS details will not match the current setup and the BIOS values will no longer apply and will either fail or become unstable. I'd recommend a CMOS reset when installing or changing any hardware that has a BIOS value which covers just about everything short of inserting a neon light."
Clear CMOS:
1.) immediately following a new build, before power is applied
2.) immediately following any hardware change/upgrade
3.) after flashing BIOS