If I understand the requirements correctly, you currently need to update the virus definition file from the [login to view URL] website. Instead of using the [login to view URL] file, which adds a layer of complexity, I would recommend having the .NET code download the necessary definition file (either the "[login to view URL]" or "[login to view URL]") directly from the website and saving it in whichever folder the antivirus software uses to store the definition files.
Implementing this method should only take a day or so to program and test. I have put the delivery time as 7 days in case you still wish to use the more complicated method of going through freshclam.exe. Please feel free to message me with any questions or concerns. A full resume is available upon request. I look forward to discussing this job with you further.
Thank You