I've had similar issues with large archives getting corrupted when I copied them around, it turned out to be a bad stick of memory.
Smaller files seemed to copy fine, but when I copied a big file (DVD Iso) from one drive to another they would occasionally get corrupted
There is a freeware windows memory checker
Here. If it picks up errors you definitely have a problem but it is not 100% reliable at finding bad ram. To be sure you should download a bootable CD image & use that.
When I had my issues the windows mem test app did find errors tho...