|"Original Op.Sys. is XP_Pro"|
Do you mean the disc you plan on using is the very 1st release of XP Pro with no service packs? But the system is currently running XP Pro w/SP2? If that's the case, you will have to completely wipe the drive & start from scratch. As stated above, make sure to back up all your important documents 1st. And be aware that some programs require that SP1, SP2, or SP3 be installed, so check the software requirements before doing anything else.