Simple Secure Free Knowledge Base

I have been wading through a ton of sites looking for the perfect product, but I'm not finding something that fits quite right. While I continue to keep searching, does anyone have any ideas?

Here are my requirements:

1. It must be web-based preferably able to run on IIS and use ASP. A selfcontained solution would also be acceptable.

2. I want to require a log in before any content is presented to the user. At a minimum, this would be a private knowledge base for myself to maintain configuration settings for projects I work on.

3. I'm fine with text articles. I would like to be able to add attachments. If I could have some limited formating capabilities - bold, italics, underline - that would be great.

4. I would love to be able to limit the availability of content based on internal groups I set up with granularity for view, edit, attach, etc.

I do not mind doing a little work with the software, but I keep downloading products that end up with poor documentation, are not complete or do not promise the functionality advertised.

Are there products of quality that meet the above requirements and at an affordable price, that base article security on an AD group?

Now that I've changed jobs, I'm generating knowledge that I'm having a heck of a time managing with flat files. I've tried Outlook, but the journal is not a great function and it is not available through OWA. I would rather an independant web-based solution.

Please help! %)