if this is an older pc with ide drives. the hard drive has to be the master on ide0 and the cd-rom the slave drive. if this is a newer rig with sata drives. make sure the sata port set to achi or ide. if it wont install in achi mode set your port to ide. also if your xp disk was a download from torrents is may be virus or not a good one. try getting the iso from digitial rivers.