Though I agree that if you do not have a good system spec, transformation packs are better not tried. Still as I have used all the three Transformation packs named here, I would recommend that you go for the one from CrystalXp.net. That seems to be the lightest on system resources. Just remember to install and uninstall it in the Safe Mode. This is the only pack which will give you back your Old XP Look once you get it out of your system. Rest two are not that forgiving and I have found that system lags a lot even after uninstalling the other two.