So if you would read up Vista has a feature called SuperFetch where it takes your most often used applications and stores them in your unused memory, therefore next time you launch that application it will launch in half the time.  Why do you want free memory laying around when it can be used, if an application needs to use that memory Vista lets it, the memory is not marked as used its marked as free to other applications but the memory manager keeps real free memory at a higher priority.

Free memory gets used first, Cache memory second, and Paging third. 

Thats the jist of this very complex feature that does an astounding job of increasing startup times of your most common applications.