Since you already formatted the Drive and nothing is on the disk you might want to start from the begining.
The bios are probably looking for A:> because your boot drive order is probably set as A>, C, CDrom then SCSI
Change it so that the boot priority goes to CDROM as the first selection. You can turn overclocking off here or even restore BIOS values to Factory default if you feel that it is needed. (your choice) Then, Save Bios's info Put the XP pro Installation CD in the Drive and reboot. When it reboots the first choice you will get is "to boot from CD ROm drive hit (whatever key it says)............." just tap the key and let XP go through the setup. If you have XP Pro full install you might want to repartition and reformat (NTFS) and install XP. If you have an upgrade version, have another previous Windows CD ready for the upgrade compliance check. If you need further asistance Email me at mtnman1@charter.net and I will be glad to help with any further problems.