My friend computer is very slow, the computer name is AMD mobile Durdon 256MB ram and 550 MHz. he down loaded window xp what’s the problem for this case?

The information that you provide is nowhere near adequate to make good suggestions on what your friend's problem is.

The system is fairly old, so if your friend is trying to run the latest games and programs, it will be quite slow.

You really must provide more information.

Did the computer become slower when Windows XP was installed? If so, then that would be because the machine won't be able to run Windows XP very well.

Please be more specific with the problem. Let us know exactly what is running slow, what was changed on the computer before it became slow, and what you expect the machine to do.

There can be many reasons for a computer slowdown. Most often it can be because of spyware/virus infections, esp if its been used for internet.
If the HDD is clogged with unwanted stuff/ the drive is fragmented, it would need a disk cleanup and defrag to improve the speed.
Too many items loading at boot/too less RAM can be another cause.

The system specs are barely adequate for XP. I suggest you upgrade the system.

Either upgrade the system or downgrade the OS. Memory is the main thing to expand but processor speed, drive speed, etc. all have an effect, too.