For some weeks, Firefox and Windows were failing. Consistently a ~400 MB file download had bad bytes, mostly at different locations. SMART monitoring said disks were okay. POST didn't complain. Old ASUS mobo, AMD processor, Windows XP Pro SP3 boot partition, Firefox 35.0.1. Solution: Booted into BIOS options, changed nothing, saved the result, computer now stable. Likely cause: A bad byte somewhere in the BIOS ROM.