i have pc with vista installed now iwish to change that into windows xp ,i tried it but some error msg shown can you help me

You'll need to find several things. First is a disk wipe software program - usually takes a good while to wipe a drive and all the boot and MBR stuff. Check out tucows.com for it. Or if you have a Ubuntu OS disk - load it and do the disk wipe/partitioning or prep to install the OS but quite before the actual OS installs.

Once that is done be sure to have all the drivers for the XP use on this PC. If you don't windows may be there but will not operate correctly and will prompt you for the proper drivers for the mother board.

Go to the manufacture site and read up if they have the XP drives for the rebuild. Else you may be stuck with the Vista OS - BEEN THERE! And that sucks! The way around that is to go to the Win 7 OS an maybe the business version for the XP duality.

Do the homework!

Hi, would you enlighten me on what error message appears that does not allow you to proceed with installation?

I reckon you are doing an install while on windows vista. And this type of downgrade is usually not permitted on some machines. I would recommend that you do a fresh install in which you will have to format the whole drive to install a new copy of windows. This requires you to backup as most files will be wiped out.