First let me point out that this has never occurred when booting Windows. So I don't think it's damage on the hardware.
So, I'm booting my Inspiron 1501 laptop running Debian, and the XDM (X Display Manager) loads the X Window System and it tries to display the login screen. Problem is, all I see is vertical stripes that are either green, blue or red (on a black background). To overcome the problem I do one of the follwing:
1: Enter my credentials, hit enter, the Window Manager loads (which happens to be Awesome), and the display is back to normal.
2: Close the lid to the point where the screen switches off, and then open it again. It's back to normal, and I can see the login screen.
In other words, it's not really a problem, I just want to know what is going on.
Earlier I used the GDM (Gnome Display Manager) and this problem didn't happen. I think it is somehow related to display mode, but I'm not sure.
Also, one final thing that might relate to the same problem. Sometimes, the first thing I see after the lines disappear is some distorted Windows 7 desktop :D. This happens if I rebooted my computer from Windows 7 to go into Debian. It seems like unflushed data in the video cards framebuffer.
Any clues? :)