Thank you all for responding to my question.
Unfortunatelly I cannot find any BIOS remap option. I updated BIOS to the latest version provided by HP. I tried to remove all the mirrors or repos and try to update kernel but in vain.
The strange thing I guess is the following fact:
H/W path Device Class Description
system HP Compaq dc7600 Small Form Factor (EC837ET#UUW)
/0 bus 09F8h
/0/1 memory 128KiB BIOS
/0/5 processor Intel(R) Pentium(R) 4 CPU 3.00GHz
/0/5/7 memory 28KiB L1 cache
/0/5/8 memory 2MiB L2 cache
/0/6 processor CPU [empty]
/0/36 memory System Memory
/0/36/0 memory 1GiB DIMM DDR2 Synchronous 667 MHz (1.5 ns)
/0/36/1 memory 1GiB DIMM DDR2 Synchronous 667 MHz (1.5 ns)
/0/36/2 memory 1GiB DIMM DDR2 Synchronous 667 MHz (1.5 ns)
/0/36/3 memory 1GiB DIMM DDR2 Synchronous 667 MHz (1.5 ns)
/0/37 memory Flash Memory
/0/37/0 memory 1MiB Chip FLASH Non-volatile
/0/100 bridge 82945G/GZ/P/PL Memory Controller Hub
/0/100/2 display 82945G/GZ Integrated Graphics Controller[/code][Moderator edit: Added [i]code[/i] tags to preserve formatting.]
So CentOS shows that there are 4 x 1GB chips installed but only use 3261 MB.
I also deactivated soundcard from bios, paralel and serial ports, also searched for any option to remove or disable memory for videocard but such an option does not exist.
In the end, I think I will only use 3261 MB of 4 Gb and that's all because I see no other option.
I am planning to buy a new hard disk also and maybe a clean install of the system could solve this problem.
Support for the other architectures (X86_64, IA-64, and PowerPC)