Your post was an interesting challenge so I wanted to see if I could find a solution. I tried creating a virtual directory mapped to a UNC path setting the credentials with a domain account that had the correct permissions. I was running into an issue when the upload.asp page was hosted on the local web server and trying to access the virtual directory setup to the UNC share.
Here is a workaround that seems to fit your requirements.
1) I created a domain user called UploadSample with some random password.
2) I created a folder on a remote server called UploadSample. I shared the folder and granted the Domain\UploadSample user modify permissions to the remote folder. + I also granted Administrators / SYSTEM full control.
3) In IIS Manager, I created a virtual directory called "UploadSample" mapped to the remote UNC path. Since this is a static user, the anonymous login can be set and not protected using Windows Authentication. This is the only vdir set to anonymous. I set the anonymous user to a static domain account (Domain\UploadSample).
4) Since you are using ASP, I downloaded http://www.freeaspupload.net/freeaspupload/download.asp and extracted uploadtester.asp and freeASPUpload.asp to the Remote Share I created in step 2. The upload program files are being hosted on the remote share. I fixed the variable used by the program. Here is the line you need to correct. uploadsDirVar = "\\DomainFileServer\UploadSample", this variable is in uploadTester.asp.
5) I made sure the rest of the site required Windows Authentication.
6) I hit http://WebServerIPAddress/UploadSample/UploadTester.asp and tested uploading files with no issues.
A few other tricks I did, the UploadSample folder does not have a default document, so when someone goes to http://WebServerIPAddress/UploadSample/, they'll get a 403 error since no default documents are set. You could even have some logic to redirect them else where. In your application, you link directly to http://WebServerIPAddress/UploadSample/UploadTester.asp For reference, the application pool was set to use Network Service. Hope this is helpful, it was an interesting challenge.
Steve Schofield
Windows Server MVP - IIS
http://weblogs.asp.net/steveschofield
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