If a new format or a defrag seems to temporarily fix things then you are losing sectors.

The firmware on a hard drive will map out dead sectors and replace them with one of a number of spare good ones. Once these run out your data goes funny.

There are sufficient of these that if you run out your disc is dying in a fairly tragic way anway.

Right-click on the My Computer icon, select Manage, then expand Event Viewer then click on System - that should show if there is anything in the event logs for disk issues.

Out of interest, do they have broadband and any anti-virus and firewall software installed?

Right-click on the My Computer icon, select Manage, then expand Event Viewer then click on System - that should show if there is anything in the event logs for disk issues.

Out of interest, do they have broadband and any anti-virus and firewall software installed?

It won't show if the disk is doing best efforts to hide it but not quite managing and losing data