I get this exception when my application wants to delete a file.

the programm load a dll-file from a server and writes the stream into
a file with this code:

HttpWebRequest wr =(HttpWebRequest)WebRequest.Create
("http://localhost/Server/Tools/"+strFileName);
HttpWebResponse ws = (HttpWebResponse)wr.GetResponse();
Stream str = ws.GetResponseStream();
...
FileStream fstr = new FileStream(strFileName,
FileMode.Create,FileAccess.Write);
fstr.Write(inBuf, 0, bytesRead);
str.Close();
fstr.Close();

but when I start this function a second time (and the session is also
running), I get the exception.

so what can I do? how can I stopp using the dll-file to delete it?