Hit ctrl-alt-delete, go to the processes tab and look for processes that are
consistently showing high numbers. You may have applications that are
either not compatible with XP or are in conflict with other things on the
system. Also, make sure your antivirus software is up to date and run a
virus scan and virus can cause such issue.
You might also want to check for adware on your system that might
accidentally have installed. Go to www.lavasoftusa.com and download,
install and run Ad Aware.