I installed Debian(I think it was called Etch) on a Dell P3 laptop and the display was not right. It looked as if it was trying to display two identical images on top of each other. I took it to my local genius kids store and the guy said,"do you need 3-d?" Not thinking I did since I was interested in the music angle I said no. He then typed at the speed of light for about 10 seconds and said, "there! fixed!" I asked what he did and I think he said something about a display driver. I'm not sure. He spun on his heel and disappeared into the back room before I could waste any more of his time.

My question is; what did he do? and how do I fix this myself in the future;

Which video card have you got? ATI, nvidia or Intel? As 3d was mentioned, it would appear that the proprietary driver was replaced with the open source driver if not simply VESA.