Fan error during boot-up

Power supply, that can be the cause. Did you try disconnecting few devices to check if the CPU fan has enough power to turn itself on. Usually motherboard will have two connectors for the CPU fan, you may try connecting it directly to the power supply. Also Check through the BIOS and see if there is a setting to disable the CPU Fan monitor (Hardware Monitor??) It should be where the temps and voltages are. The BIOS in most computers has an option to monitor a CPU fan. If none exists, then the option needs to be disabled. I am assuming the error is occuring during the BIOS stage of bootup. If the error occurs in Windows, it may be that some software has been installed to monitor a fan that doesn't exist. Sometimes monitoring software comes on the motherboard CD and someone may install it, thinking it is a driver. It is not necessary. Good luck!

What is BIOS and where can I find it?