Vista implements a new type of memory managment called SuperFetch. 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 enables you give priority to important programs that you use infrequently.
Therefore, if you leave your computer to go to lunch and a memory-intensive background task causes the code and data from your active applications to be evicted from memory while you‘re gone, SuperFetch can often bring all or most of it back into memory before you return.