Compaq EVO N600C Password-protected CMOS - Lost password

Try pulling the batteries for a few hours. You could be lucky.

If resetting CMOS batt does not do the trick you will need a new security chip installed.. This is the only option.. (Not familiar w/make and model to tell whether CMOS and BIOS reset will accomplish the task..

A. By Using the Motherboard Jumper:
In most motherboards CMOS battery is soldered, which makes it difficult to remove the battery. In this case we use another method.
Almost all motherboards contain a jumper that can clear all CMOS settings along with the BIOS password. The location of this jumper varies depending upon the motherboard brand. You should read your motherboard manual to check its location. If you don't have the manual then look for the jumpers near the CMOS battery. Most of the manufacturer label the jumper as CLR, CLEAR, CLEAR CMOS, etc.
When you find the jumper, look carefully. There will be 3 pins and the jumper will be joining the center pin to either left or right pin. What you need to do, is remove the jumper and join the center pin to the opposite pin. e.g. if the jumper joins center pin to left pin, then remove it and join center pin to right pin. Now wait for a few seconds and then again remove the jumper and join the center pin to left pin.
Make sure to turn the PC off before opening the cabinet and resetting the jumper.

B. By Using Software:

I have found that BIOS/CMOS Password Recovery Tool is the most effective.:
BIOS/CMOS Password Recovery Tool is a program that works instantly to remove any lost or forgotten BIOS/CMOS password. Simply boot your PC to DOS and execute the program, and get access to forgotten BIOS/CMOS passwords in just seconds
BIOS/CMOS Password Recovery Tool Service: http://www.biospasswordrecovery.com/