5s update/restore fail using itunes

i backed up my 5s with itunes so i could update my phone from 7.1 to 8.3; The update/restore failed; I have tried repeatedly; my phone is not functional, and I can't even restore to factory settings; In troubleshooting I have done the following: used most recent itunes, disabled antivirus software -still failing. My computer is running windows xp -is that why it is failing? I don't know what to do. I need to get my phone back to functioning.