NMGLUGers & Mike, There are several Youtube videos on this problem. Maybe just a re-seating of the heat sink on the processor, maybe check that the fan is/is not running. Use 'sensors' to see what the operating temperature is. If you want a bigger machine, fine, I do not have a suggestion. If the machine is equipped like the one in the second email, it is good for a few more years of use with Ubuntu or other Debian based OS. At 12 years old it might be good to look under the keyboard for dust and physical status of fan - blowing out accumulated dust may solve the issue. Also check the bios settings to turn off the warning if you think the fan is intact and working. If it looks damaged or there is heat damage nearby you can choose whether to try a fan and heat sink replacement or save up for that bigger machine.
Good luck, thank you, Ted P