Windows Aero rating

ok didn't know where else to put this but anyway if you need to move it feel free but i play a game called rappelz some may have played it it is designed to run on xp so my specs for my laptop are over in every way but here lately it has been lagging really bad checked my connection 1 MB is avg download rate so i moved on checked for viruses none so i did the little test that vista lets you do about your system with the sub scores and the base score well my windows aero test didn't go so well it dropped from 4.0 to 2.9 my question is what affects that rating i think that might be the problem cause if i changed nothing then why would the rating drop and that bad any help would be greatful thanx