diskeeper is very good(xp`s own defrag is a cut down version of this software)
some people have lots of problems with norton(symantec)software,some dont.
i would remove it and see if things speed up again.
type "uninstall" and tick the boxes for any norton software you have like systemworks into the "Search all available knowledge bases" box on this page for guides to completely uninstall any norton software you have.click here follow instructions.
if things speed up and you have a norton firewall and av and removed it,then replace with something like avg and zonealarm.