使用WebClient的uploadfile方法上传文件出现500错误

源代码:
this.txtUploadFile.Text=this.openFileDialog1.FileName;
string filepath=this.txtUploadFile.Text.Trim();
string server="http://localhost/sheetNew"+Path.GetFileName(filepath);
WebClient client=new WebClient();
byte[] buff=client.UploadFile(server,"PUT",filepath);

总是出现远程服务器(500) 内部服务器错误
请高手指点
虚拟目录已经具有写入权限,操作系统Windows XP Professional