Firstly there is no reason at all to flash the BIOS - you can only mess the system up totally if you get it wrong.
Put all your drivers onto a CD apart from the SATA driver which you will need onm a floppy.
Put your XP Pro disk in and boot the machine. Choose fresh install and when prompted press F6 to load the SATA driver.
When this is done XP will continue to load.
Once install is complete then go to Device Manager and load any additional drivers from the CD that are required (look for yellow exclamation marks against entries, this indicates no driver loaded).
If your SATA connections are not in use then it seems likely you have an EIDE hard drive and not a SATA.