This is a computer I sold about a year ago. It's been working fine up until about a month ago when it wouldn't load up to Windows. Turns out the HD had many bad sectors. I managed to fix some of them, backed up the data, exchanged the hard disk and reinstalled everything on a new hard disk. Customer goes home happy.
A few days later, they come back saying the computer doesn't load up again. I check it and it has some bad sectors again. I managed to fix all the bad sectors, ghost the HD and decide to give them a new hard disk. I put the ghost on the new HD and send them on their merry way.
2 days later they are back here, same problem. I am checking it right now, and it has bad sectors once again.

So 3 hard disks, one a year old and 2 brand new ones get bad sectors within days of being used. They are WD SATA drives. I can't believe this is pure coincidence. Something must be causing this, but what? They have a UPS, which should protect it. I'd think that if the UPS was faulty and was zapping the computer with some high voltage, the motherboard would be the first thing to go.
So any suggestions to what can cause this?