I just wanted to add that I had exactly this problem. I had previously managed to install LWJGL and run some simple stuff, when it suddenly stopped working, giving me the error "Could not find a valid pixel format" even though I was clearly using a pixel format that should be ok. I should add that my graphics card and other OpenGL apps seemed to working fine.
Solution: I downloaded and installed new drivers for the graphics card anyway. Now it works.
I don't know if this is relevant, but the problem occurred after I used the TV-out thing on my graphics card for the first time.