Game keeps crashing my computer

[contempted]

I run civ 4 and bts on Steam with a very old Windows 7 machine. Lately the game, especially BTS keeps shutting off my computer. Sometimes upon rebooting, I see a message that my computer was shut down due to overheating. I looked inside, there didn't see any dust and the computer is in an area with decent ventilation. Is this something that happens with older computers? Anyone who can advise me, thanks.