well the registry is the most imp part of Xp..it has all the info about the softs installed ...file extensions..handlers..behaviours..etc..in short it is totally integrated in XP ..so u cant reinstall it....also its not a visible part..ie ther is no folder / exe etc named registry.....
so if u have any probs in registry..try some expert softs like TweakNow RegCleaner Pro or regfixer pro etc... but u want to reinsall reg.....so 1 u can uninstall all the apps (all means every MSN , WMP , IE etc) reboot ..then run any reg tool and deleted any unwanted entries ..so that u will get a Empty registry with bare min entries ..and u can install the apps again......
or Reinstall the XP ......