Different people will give different answers, some claim that the regular switching on/off of a PC wears the components due to thermal expansion and contraction as it warms and cools, thus shortening the life of the PC. Others like myself will say that I've never had a PC fail for this reason and can't see the point of paying Powergen et al for the privilege of running an idle PC. I've set the power management to what I find suitable, but still turn off at the end of the day. This also cuts out any chance of voltage spike damage in a storm, I do have a surge protector, but still feel safer with the machine off.
CPU fan life depends a lot on conditions, in a clean room they can last for years, carpeted dusty rooms can reduce life span drastically.