Was given a HP Compaq DC5800 with Core 2 Duo E4600 that was overheating after replacing CMOS battery. 

Prior owner was fed up with it and bought a new system and gave it to me for disposal. I figured initially that the heatsink must be clogged with dust. opened the case and it was surprisingly clean inside. Then removed heatsink to apply new thermal compound after removing old compound. Booted up again and quickly to 60C at idle and tops out around 72C. Didnt feel much airflow going past the heatsink. With speedfan installed I tried to raise the CPU fan speed from 35% to 100% and no change, still spinning slow.

Noticed the BIOS was 1.55 and there was a version 1.60. Flashed bios to 1.60 and went into BIOS to configure and thats when I found a FAN control in the BIOS with modes. I set it to 4 bars of like 6 bars for the mode which was a decent speed for the CPU fan. The CPU temp dove quickly to 48C and then came to rest around 43C.

Looked into the CPU support for this system and saw that the Q33 Express chipset supported the newer Pentium E5400 2.7Ghz that I had available from a dead eMachine. Removed heatsink again, removed thermal compound and swapped out the E4600 Core 2 Duo 2.4Ghz 2MB L2 Cache CPU for the E5400 Pentium 2.7Ghz 2MB L2 Cache CPU that has a better benchmark. Applied new thermal compound once again. System runs cooler with the Pentium E5400 than the Core 2 Duo E4600 at 38C at idle and when running Y-crunch multi-threaded benchmark it tops off at 52C.

Sharing this here as for someone else MIGHT run into this overheat by BIOS design problem with the DC5800 or other similar HP Compaq Models. I cant believe that HP Compaq would engineer a system that would remain at a slow speed for a CPU fan and allow for the CPU to run into the 72C range without the system ramping up the CPU fan speed to try to cool it. I shut the system down after I saw 72C to avoid cooking the CPU. I wasnt sure if it would thermal throttle to try to protect itself or if it will just cook itself to DEATH.

Not a bad FREE computer, and will likely give it to my daughter as an upgrade since she is running a Celeron D 335 single-core with XP.

Next time someone SAYS their system is overheating after replacing the CMOS battery, I know right were to point them! Fan Mode in BIOS in which default is the lowest CPU Fan Speed which is stupid!  I would think default would be 50% speed. I set it to 50% speed as for 100% makes it sound like a Server very NOISY and 50% you can hear the fan but its not that bad for fan noise.