LaTeX.org

Thanks. The error about xcolor and ulem doesn't appear any longer but the errors while exporting a pdf file are still the same.

I've reinstalled my PC (and obviously LyX) and now I have problems using LyX. Every time i open some one of my LyX files (*.lyx) I get the following error:
Changes will not be highlighted in LaTeX output when using pdflatex, because xcolor and ulem are not installed. Please install both packages or ...