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Overclocking is dangerous, and can lead to hardware failures. Try this at your own risk!

The HP DC5000 (no AGP, no SATA) or D530 (SATA + AGP) computers have no overclocking features in the BIOS, yet there are software methods that provide successful overclocks. These motherboards use the Intel 865 chipset. Using CPUFSB downloaded from http://www.cpu-cool.de/index.html you can overclock the processor easily. I used CPU-Z to verify and GeekBench to view the performance benefits. I was able to push a Pentium 4 Prescott 2.8GHz CPU to 3.1Ghz reliably. If I pushed to 3.2GHz there would be a loud noise from the motherboard (I do not recommend going this high on a 2.8GHz).

Background Info:
The PLL chip used in these systems is the ICS 952606. You can verify this by looking near the AGP connector. The CPUFSB program does not have HP's intel motherboard in the list, but it will suffice to use an ASUS P4P800(-VM) and change the PLL to 952606 from the list.

Steps (FOR HP DC5000 or D530):

1) Download CPUFSB and install.

2) Load the program and under Mainboard Manufacturer choose Asus.

3) Now under Mainboard type, select "P4P800(-VM)"

4) PLL Manufacturer should be ICS and the PLL type should be changed to 952606.

5) Now we are ready to overclock.

6) Using a program like CPU-Z (http://www.cpuid.com/cpuz.php) you can verify the clock changes in real-time.

7) Back in CPUFSB, click on "Fine Tuning"

8) Read the warning signs and press Ok if you understand the dangers.

9) Now, you'll see a window with +60 +10 + - -10 +60 (these are the increments and decrements in MHz that you can do)

10) Try adding 10 or 60Mhz and then press Freq Set. CPU-Z should display the change in the clock speed.

11) Play with the speeds but try not to go too high, since you could possibly cause damage to the motherboard's clock circuitry.

Enjoy!