ASP Net System.io. file and directory operations

I have tried logging in on my PC as (domain) Administrator and it does not work either (in the same way as my user fails) so my feeling is the problem is passing the logon from IIS to the file server (although why it works logged on to the server I have no idea!).  When I check the event viewer (security) on the file Server it has 'ANONYMOUS LOGIN', NTLM (when I run from a PC)
However when I am logged in to the (IIS & SQL) Server console th event viewer says 'Domain\Administrator', kerberos,  ...

In answer to your point ASP.NET is a local user (I don't know if it is a big deal to change it to a Domain user?) so I cannot give it access.  Do I need to make ASP.NET a domain user?  shall this jepardise my other running asp pages?

Is it an intranet site? Does it work based on authentication? Is authentication windows based or forms based?

Sorry for the Delay never got a mail ...,

It is an intranet site, the authentication works to SQL (Inserts, Updates, etc) but it is the SQL Server, i.e. IIS rus on the SQL Server and this is where I have deplyed the site (Project).  To be honest I am not entirely sure how the authentication works (or doesn't in this case).

We have other sites on the server that work fine (for years) but they are basically SQL data updates.

If you are not using any authentication then i think anonymous access is being used on the IIS. There is an IIS user as well so you may want to grant it permissions for files. A better solution is that if your users are on domain then use windows authentication and then grant access to different user accounts.

Sorry sorry, it IS Win authentication (even checked on IIS) I just disengaged my brain there.
Sorry for the delay but I thought I woudl try thinking for myself a bit but to no avail!  I did have the aspx page (/code) in my colleages project SO I just created a new blank Visual Studio Web site and imported the aspx page and aspx.vb code. Just got the same error unfortunately. Attached is my Web.config, in case there is something wrong there.
As I said my ASP.NET user is Local to the Server (not a domain user, rightly or wrongly?) so I cannot give it permission to the Network share.
i did read somewhere in my research that I am better off using mapped drives than UNC's would you agree with that?

If using a mapped drive solves your problem then why not. Give it a try.

I have tried using a mappped drive, but it does not work either.  I did try this earlier in the process and it did not work then either.
on my pc I get "An application error occurred on the server" ... try it on the server or 'mess about' with custom errors.  TRied it on the Server and it works!! (as it did on the UNC path).
My thinking is it must be the IIS config on the Server, any thoughts?
It would be a bit of a pain but would I be better (easier!) to deploy this as a windows form?

I prefer WinForms because it gives you a lot more functionality, access to system resources (including file system) and many other benefits. And with ClickOnce now, it is very easy to deploy and auto update applications as well.

I have my (interested!) consultant looking at this, if he finds a cure we shall post it otherwise I shall accept your solution, I guess half points?

I've fixed it!
What I did was give the (AD Computer) SQL Server (the one that the IIS this proiject runs on) full access to the network share AND changed the line in Wed.config to:
<identity impersonate="false" />
and that worked.  
Interestingly my consultant did not do the Server access, it worked with him, when he gave the network service access to the file share.
I shall give you the points as discussed but hopefully anyone with the same problem shall read this bit as well.  Thanks.