Lysander said:
So all this cached memory is already in use helping apps that are already open?
The memory in use as cache will contain files you recently accessed (and therefore could be expected to need again), bits of the operating system that aren't in use right now but have been at some point or are likely to be needed later and, most importantly, programs that Windows has determined you use a lot and are likely to use in the current time period.
The system can overwrite any of the cache at any time when a program actually wants to reserve more memory for itself, with no loss of data. That's why it's not listed as in use on the green bar. While it's not exactly free, it's not reserved for anything either and is not locked out. It just gets used to maximise performance rather than sitting there doing nothing.
Regarding harddisk activity on the login screen, the suggestion above that it's due to Windows pre-loading applications into the cache is probably right. Also, as you're on the login screen and therefore not doing anything with the system right now, Windows can prioritise housekeeping tasks like updating the search index.
On a similar note, Vista is now aware of multimedia applications. This means that when you're running a DVD player, the system knows you're doing something which requires real-time uninterrupted decoding and can prioritise that program (and background tasks) accordingly. This should mean you're much less likely to see your video skip during playback.
It's all about being smart and making the maximum use of resources.
