Texmaker is just a TeX editor that uses some packages of existing ditribution of Latex (MikTex, Texlive etc).
My guess is that either your Windows XP Professional is using a Latex distro that does not have xfrac package included/installed (latest versions of TexLive do have xfrac) or your texmaker can't use \usepackage{xfrac} or your site tex renderer does not have xfrac package included/installed or if you are using tex filter the latex preamble setting (in settings of tex filter) does not have \usepackage{xfrac}... Latex can be complex to use sometimes...
What exactly did you mean by "I did that man but its not working at all" - ?