SQL Server 2005 Workgroup - Memory filled up even the application is not running. Help !

Hi

I am using SQL Server 2005 Workgroup with an IIS applicationon the same server. SQL Server is installed with all the defaultsettings. I haven't changed anything. For example:

Memory usage = Dynamic (0 to 4 GB - Max RAM )
Max worker threads = 255
Database Auto Close = Off (unchecked)
etc.

Theproblem is that memory usage is growing time after time and reaches toalmost 3 GB and even now when the application is not running on thisserver, my SQLSERVR.exe memory usage is stuck on that increased value.I must have to restart the SQL service to get it down.

Why is this happening ?

Please Help.