saving a file (FSO)

Still got a brand new prob about where saving my file.
by default the file is going to be saves into the 'document' folder declared
in the path.
but it is a physical path and not virtual. -->
C:\..\inetpub\wwwroot\document\ID1_ID2.txt
what I should do is saving the file on the server
so what I did is create a new var with the physical path of the the server
IP and so on.
and what I get is this error:
Server.MapPath() error 'ASP 0172 : 80004005'

Invalid Path

The Path parameter for the MapPath method must be a virtual path. A physical
path was used.

What can I do? cheers