Dear you should provide complete related information about this. Like the source and destination file path will be like phyiscal path not virtual path.
Just to add
Server.MapPath("/root/myfolder") will give the physical path of the folder from starting from the driver letter that can be used in place of "Source File Path" and "Destination File Path".
So the ammended code snippet may look like
System.IO.File.Copy(Server.MapPath("/root/source/a.xml"), Server.MapPath("/root/destination/b.xml"));
Thank you.
Lakhan, please do not mind, just an effort to provide complete info :)
Cheers