Looking at Possibilities
What size images are we talking about?
Is the camera set to take full size files?
Ideal will be the maximum size the camera can produce and the .jpg quality set to the best.
Cameras have two to three .jpg quality settings.
The main ways the photo files get to the computer is via a USB cable connected between the camera and the computer.
This method also uses some type of software to do the transfer.
Another way is to use a Card Reader.
The memory card is inserted into the card reader and read directly into the computer.
At that point do we know it got into the computer OK?
You have to look at them to know it is really OK.
Some type of software has to be used to look at the photos.
If we just look at the photos, there should not be any change to the image.
If we load the photo into a program and then save them back to the computer, we should always do a Save-As under a different name.
That way we still have the original and now a copy.
The copy will not be exactly like the original due to .jpg quality settings in the software. The software will let you choose the .jpg quality setting before you do a Save or Save-As. If the quality setting is set low, we can degrade the image. We should set it to 100%.
At this point we know we have a good original on the computer.
If we did a Save-As, we have a good original and a good copy.
Now the photos stay on the computer for a period of time.
Then later we decide to look at them again.
We discover the bottom half of the images have deteriorated.
How did that happen?
1. The disk is corrupting files.
...But it would also be corrupting critical operating system files too.
...That would cause crashes and blue screen locking of the computer.
2. When we took the first or second look at the images, the program we are using is corrupting the images and somehow writing them back to the hard disk.
3. We have some rogue software on the computer (virus, Trojan horse, etc)
To narrow down the possible cause, I suggest you write a copy of the photo files to a CD or USB memory device right after you load them onto the computer; before you even look at them.
Then look at the photos you downloaded and if they look OK, copy the photo files again to a CD or USB memory device.
If you later find the files have corrupted while in the computer, you should look at both of the CDs or USB memory devices (using a different computer). That will tell us if the problem is to do with getting the photos into the computer or with the programs we are using to look at the photos, or the computer is somehow corrupting the photo image files.