Thanks David, the hdd's brand new, though I think my brother may have tried to install windows already, so will probably have to format first, then:
I think what I have to do is copy the driver files onto a floppy (+find a floppy drive from somewhere), then insert the XP boot disk, hit F6 to install SCSi and Raid drivers, then put the disk in, once these are installed the computer should be able to detect the HDD, and hopefully it will all work... fingers crossed, now to find a floppy drive....hmm...
Apparently even though I'm not using RAID, i still need the drivers installed to use a SATA drive.