Windows uses quite a lot of memory when idling, and will have memory allocated ready for certain processes. Windows is always busy, it's never "idling" - if it's not running user tasks (application programs), it's still running many system tasks.
The whole memory space includes real RAM and virtual RAM (which is actually the pageing file on the hard drive). It them balances the use or these two "sections" of memory, dependant on how much real RAM is fitted. It uses more real RAM when there is some, and more virtual RAM when there's less real RAM.