Let me guess: Your users have been working, and things have been going great. Then, over time the machine starts to, oh...let's call it "deteriorate." You load more applications, and things get slower. You add a patch, and some applications start acting funny—things start crashing and users are rebooting more.
And you know why.
Because Windows doesn't do a really great job at isolating applications and keeping them from kicking each other in the shins. If you have DogFoodMaker 2.0 and CatFoodMaker 3.0 and they each use a DLL named FOOD.DLL, which application is going to use which file? And what if CatFoodMaker 3.0's DLL doesn't work quite right with DogFoodMaker 2.0?
Chaos and performance ...