What brand of hard drive are you using?
You could write zero's to the drive bringing it back to almost like new condition using the floppy that came with your drive.
If you don't have the floppy go to the mfgs. website and find the utility that writes zero's and download it. Of course this will take a few hours depending on the size of the drive, But it will be clean.
Or you can go here
and download a Win. 98 boot disk.
I think you need to download the red one that will create a boot disk in windows.
Boot to this newly made Win98 floppy selecting start computer without CD-Rom support.
At the A:> prompt type fdisk /mbr.......Enter.....This will be a quick operation you will probably think it didn't do anything but it did.
After that type fdisk..........Enter
At the prompt do you want to enable large disk support select yes.
Now you want to delete any partitions that are on the drive by selectin option 3.
Then selecting the partitions you want to delete.
Once this is done you should be able to change your boot sequence in the bios to boot from CD first and save. Insert the Win. XP CD into the drive and reboot the computer to start the install.