According to an article in the Seattle Times, "Apple's software runs on computers with half the memory required for a Vista machine." Just goes to show you the trouble with the system as compared to that of the Mac's. The operating system's occupation of so much of the memory in the machine is going to slow down your PC.
The article also suggested two ways to speed up Vista on your PC.
1. Turning off the "view thumbnails" feature in the control panel.
2. Making sure that your machine has enough main memory to run Vista.
You can also look on the bright side of what the operating system has to offer--Vista has way less of a chance of getting a virus than XP.