Thanks for the reply Jagaf,
I haven't had a chance to check for a virus yet, I live 700kms away from mum so it can be a little difficult to help. I generally use TeamViewer to remotely access her computer. Which might I add, uses little data. She's running Windows 7 on a 5 year old laptop. I removed the Windows 10 update (even though it says her computer could handle the upgrade, I beg to differ). She does very little on the internet, Facebook, Pinterest, general browsing and email. Her average monthly usage for the past 2 years has been now more than 4.5 gig. It isn't just the fact it's a 70Gb download, it's also that it occured in 1 day, her average DL speed is 5Mbps. That equates out to 234.48 hours, almost 10 days! I'll do a deep virus scan to see what I can come up with and have a look at her gateway to see if there have been any outside connections. If that's the case I'll just set up MAC filtering. We've already used one of the free top-ups, thanks for the reminder anyway.