15% is incorrect. The system will use what it needs, and may cache more system files in ram if it thinks it will help overall. Most versions of Windows will use between 1Gb to 3GB or ram, depending on what version and how much overall ram you have.
Applications will use more of course. On systems with less ram (3GB or less) the system will release cache when an application needs ram. On larger systems (6GB or more), the system will usually just keep its cache, using around 2-3GB of system ram.
These numbers all vary if it is a 32 bit or 64 bit system. Obviously 64 bit systems are needed to address more than 4GB of system ram.
I have a system with 12GB of ram, and even as I type this answer, my total ram usage is just below 3GB of actual "in use", but around 9GB of "Cached" ram, meaning the OS has used what it wants to cache system files. That 9GB is free for other programs, if any gets loaded. Cached ram may build up slowly as you use certain applications that load certain system files. For example, I had just done a large backup of my system before that 9GB was used as Cache. It may be residual backup data still being cached by the system (or the backup application which monitors in background)