Problem implementing my File_Upload Class - 500 points

I'm getting the following error and I'm not sure why:

Warning: filesize() [function.filesize]: stat failed for C:\WINDOWS\TEMP\php1786.tmp in C:\Documents and Settings\username\My Documents\UploadFiles.php on line 140

Here is the code that the error refers to.  The function is defined within a class.  Can someone tell me if this is a coding problem or a permissions problem?

      // Returns file size in a user friendly, readable format
      function get_file_size() {
            // Define variables to be used by function
            $temp_file_name = trim($this->temp_file_name);
            $kb = 1024;
            $mb = 1024 * $kb;
            $gb = 1024 * $mb;
            $tb = 1024 * $gb;
            
            // Check to see if temp file name is valid, if not, return error.
            // Get the file size in bytes using filesize() function.  Return a readable file size.
            if ($temp_file_name)
            {
<HERE IS LINE 140>            $size = filesize($temp_file_name);
                  if ($size < $kb)
                  {
                        $file_size = "$size Bytes";
                  }
                  elseif ($size < $mb)
                  {
                        $final = round($size/$kb,2);
                        $file_size = "$final KB";
                  }
                  elseif ($size < $gb)
                  {
                        $final = round($size/$mb,2);
                        $file_size = "$final MB";
                  }
                  elseif ($size < $tb)
                  {
                        $final = round($size/$gb,2);
                        $file_size = "$final GB";
                  }
                  else
                  {
                        $final = round($size/$tb,2);
                        $file_size = "$final TB";
                  }
            }
            else
            {
                  $file_size = "ERROR: NO FILE PASSED TO get_file_size()";
            }
            return $file_size;
      }