What about SuperFetch in Windows Vista?

SuperFetch is a new function of Windows Vista. With SuperFetch, the PC can read software and files more quickly compared with Windows XP.

In the older versions of Windows, system response may sometimes become unstable.
When the PC is not in frequent use, in order to reduce processes, the PC’s operation may slow down when processing backstage tasks (like automatic copy and anti-virus software, etc.). 

SuperFetch, the newly-added function of Windows Vista, can recognize the most commonly used software, and pre-read them so that the response rate of the system can be raised when it is first started or shift to anther user’s accounts.

As the priority of software can be set when operating backstage tasks, even a period of inactivity will not alter the response rate of the PC.