Here's a thought: try a KDE live CD (e.g., Mini-me) instead of LXDE and see if the keyboard maps correctly. If it does, install that and then add the packages for LXDE from the repos. (Yeah, I know, not a very elegant solution...)
If KDE doesn't recognize it correctly, begin to suspect something in the decoder or keyboard map. (You haven't recently replaced the keyboard, have you? If so, check your ribbon cables for proper connection.)
I used to own a Gateway 3522GZ (somewhat older than yours, I think) and never had a moment's trouble with it, but I never installed LXDE as the primary environment, always after KDE. (I used to use LXDE a lot on that old machine to do audio recording with Ardour. It really helped lower the overhead.)
Later On,
D
