I found a great little article on the web recently. It seems this guy has always been more than a little curious as to why his PC runs great initially, but then the more things he installs on it, the slower it gets. He decided to do a detailed test of his PC's speed both before and after installing specific applications..over 30 to be accurate. Here's a link to the last page of his article including the results of his testing: 
 
http://www.thepcspy.com/articles/other/what_slows_windows_down/3 
 
It always makes me feel a little bit better knowing why things happen, instead of just having them happen and being in the dark about the true cause. 
 
Enjoy. 
CH