If it is a Return To Base Warranty, I suspect there is little you can do about charges. Have you put your case to Comet in writing to their Head Office, if not do it NOW by recorded delivery rejecting the laptop as they have attempted repairs on numerous occassions. List all details in the letter and give them 28 days to provide a replacement. Keep a copy of the letter and if no satisfaction in the given time scale, take advice from Trading Standards or take legal advice. If you have taken finance on the PC, copy your letter to the finance company.