If there are any beeps when you power up, try to describe them... one short, one long, three shorts, one short <pause> one long, etc. Generally, one short beep is good. Anything else is likely a hardware problem.
That does not sound like a Windows XP message - it sound like a BIOS kind of message.
Usually there will be some other messages about the BIOS version and manufacturer which may be helpful to look up the error message.
I am not sure what you mean here:
Now I can tell you that all the USB plugs in the Computer were removed so a NEW CPU Fan could be installed. I DO NOT KNOW IF THEY WERE PUT BACK IN THE SAME PLACES ????
All the USB plugs were remove? Do you mean every device that was plugged in was unplugged and may not be plugged back into the original holes?
Start with the most simple stuff. Unplug everything except the power cord and monitor.
If you have a USB drive or printer, unplug all that stuff too - you don't need it to boot.
Fire it up with just the monitor. See what happens. Pay attention to beep codes! They are telling you something is right or something is wrong.
The next thing you "need" is a keyboard. Plug in the keyboard, power up again, see what happens.
If no go, move the KB to another plug, cycle power and try again (only 4 possible plug in places, right
When you get it to boot with monitor and KB, then figure out the mouse, and keep going until it doesn't work anymore or everything works.
Try to figure out the make and model. Desktop, laptop? If it was built from scratch and you don't know, say so to stop us from asking the question.