I have a problem, I have been searching for more than a week online and testing. I rarely post in forums. but I have gave up. I'm sorry that the post is little long.

I have an old pc, 2.4/265 celeron CPU, 160GB WD harddisk and 2x 512 RAM.
The problem begun after I changed CPU. I had two XP installation, and after changing the CPU with a used one I wasn't able to boot, both XPs installation, it restarts automatically at login screen and sometimes after logging.
I tried to boot with disabling automatic restart (from F8). I got the blue screen of death saying "DRIVER_IRQL_NOT_LESS_THAN_OR_EQUAL" and with a the file "atapi.sys" and sometimes with error code "STOP 0X0000000A".

After searching I found this file error (atapi.sys) means error in HD or CD Rom.
I removed windows and tried to install another xp or server 2003 I got the same message.
I removed most hardware devices from the box (CD rom, all pci cards, and a ram) and tried with minimum devices, I got also the same error.

I' ve checked the memory with Windows Memory diagnostic dos program and checked harddisk with HDD regenerator prog. and formatted it too :( , and no errors or bad sectors at all.

I tried linux (ubuntu) I can boot the live cd but when I try to install it (after about 70%) the system hangs (panic) .

I removed the harddisk and tried to boot with ubuntu again and install a program it hanged, which mad me think that it may not harddisk problem.

Now, when I try to install XP or server 2003, it hangs with the the same error while typing the serial (after installation of devices driver phrase in windows installation).

Any Ideas about what to do or test?

Thanks in advance.
Now I don't know what to do to figure out the cause of this problem

and after changing the CPU with a used one I wasn't able to boot

so problem all starte after you did this ,so why not assume that this the problem and change it back

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