|The first thing you try is go into the bios Setup, Save bios settings, reboot.|
If you still get the message when you boot after that, go into the bios Setup and check to see if the Time and Date are correct - if they aren't, you probably need to replace the Cmos battery on the mboard, assuming it's installed correctly with the + on the top showing when it's installed.
If the bios "thinks" the battery is too weak or dead, you will get the message every time you boot.
If the Time and Date ARE correct, then
- tell us the model of the mboard
- tell us which specific cpu you just installed - preferably it's Sxxx designation.