I've been working on digitising our family's very old home movies and I settled on ProjectSend for then distributing the files to the family. It works well, although its features are probably overkill for what you need.
Depending on your specific needs, ProjectSend may do the trick. (http://www.projectsend.org/)
It's for sharing files with specific groups of persons ("clients"), but clients can also upload some files to it's storage space.
Pretty sure you're looking for Project Send http://www.projectsend.org/
I know you want Windows Based, so you'd have to install PHP and MySQL on Windows, and configure IIS accordingly.
EDIT: I tried this out once way back, but ultimately went with Pydio as it integrates with so much more. And you can have it point to an existing filesystem. Meaning, drop a file in your network share and have it visible in Pydio in a folder you've shared with a client.
Project Send seems to be very niche to just client facing downloads/uploads. So if that's what you're after, I'd start there. Then if you want more, check out Pydio.
Have you given ProjectSend a look?
Been using it for client files going on 2 years, and it hasn't failed us yet - Best of all, even our most tech-unsavvy clients seem to figure it out without need for explanation.