how to get the unused software in the system by using c#.net

Yes, loop through all directories and add the items to the listbox.

Below code does it slightly different, it loops through specified directories and add files older than a certain date to a datatable. The datatable needs to contain at least 2 columns for file name and last accessed date.

You can use the datatable as a data source for the list box.

        ///

        /// traverse directory recursively and find 'old' files
        ///

        /// directory
        /// reference date
        /// datatable to store file names and dates
        ///
        private bool traverseDirectory(string strDirectory, DateTime dtReference, DataTable tblResults)
        {
            // find directories and process them
            string[] directories = Directory.GetDirectories(strDirectory);
            for (int cnt = 0; cnt < directories.Length; cnt++)
            {
                traverseDirectory(directories[cnt], dtReference, tblResults);
            }

            // find files and process them
            string[] files = Directory.GetFiles(strDirectory, "*.exe");
            for (int cnt=0; cnt < files.Length; cnt++)
            {
                DateTime lastaccesstime = File.GetLastAccessTime(files[cnt]);
                if(lastaccesstime<=dtReference)
                {
                    tblResults.Rows.Add(files[cnt], lastaccesstime);
                }
            }

            return true;
        }

Note: it's strongly advised to use a backgroundworker as it might be slow and the UI becomes unresponsive.

You can check the last accessed time of files using the fileinfo class

Example code:

FileInfo fi = new FileInfo("yourfile.exe");
Console.WriteLine("Last access " + fi.LastAccessTime);

Embed it in a program that recursively loops through all directories, finds exe files and check the lastaccesstime.

Note:
You might have to run the below command in a command prompt with admin privileges to enable changes of the lastaccesstime on the NTFS filesystem

fsutil behavior set disablelastaccess 0

To disable it again, change the 0 to a 1