Didnt feel much airflow going past the heatsink. Booted up again and quickly to 60C at idle and tops out around 72C. Then removed heatsink to apply new thermal compound after removing old compound.

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. 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. I set it to 4 bars of like 6 bars for the mode which was a decent speed for the CPU fan. 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 shut the system down after I saw 72C to avoid cooking the CPU. 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. 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 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.

Hp Dc5800 Specs