http://www.microsoft.com/windows/products/windowsvista/features/details/performance.mspx
Windows SuperFetch
A new memory management technology in Windows Vista, Windows SuperFetch, helps keep the computer consistently responsive to your programs by making better use of the computer's RAM. Windows SuperFetch prioritizes the programs you're currently using over background tasks and adapts to the way you work by tracking the programs you use most often and preloading these into memory. With SuperFetch, background tasks still run when the computer is idle. However, when the background task is finished, SuperFetch repopulates system memory with the data you were working with before the background task ran. Now, when you return to your desk, your programs will continue to run as efficiently as they did before you left.
http://www.microsoft.com/windows/products/windowsvista/features/details/superfetch.mspx
Windows SuperFetch enables programs and files to load much faster than they would on Windows XP–based PCs.
When you're not actively using your computer, background tasks—including automatic backup programs and antivirus scans—run when they will least disturb you. These background tasks can take up system memory space that your programs had been using. On Windows XP–based PCs, this can slow progress to a crawl when you attempt to resume work.
SuperFetch monitors which applications you use the most and preloads these into your system memory so they'll be ready when you need them. Windows Vista also runs background programs, like disk defragmenting and Windows Defender, at low priority so that they can do their job but your work always comes first.
