Try as I might I can't find many free trial practice exams for the MOUS XP certifications, either. My search indicates that http://www.customguide.com/mous_quiz.htm and http://www.certifyexpress.com/archieves/november2001.php are about the only vendors around who offers free MOUS practice exams for Office 2002/XP components.
You'll also find lots of useful pointers to practice question information in this CertCities article, but not much MOUS stuff there, alas.
The Office 2002/XP exams are pretty new, however, so all I can suggest is that you get in the habit of using your favorite search engine(s) on a regular basis and use "free MOUS practice exam" or "MOUS practice exam" as your search string. There do appear to be plenty of commercial products available for these exams for a fee, however. Here again, a search engine can help you identify your options.
This was first published in July 2002