Apache has a module that does RADIUS authentication, although it is typically used for website authentication. I'm sure it could be adapted to your needs though. Why wouldn't you just use the built in RADIUS client for Windows though? That way a client would need to authenticate for all traffic, not just web-traffic. I guess this might require some user configuration though.

You can check it out at:
http://www.freeradius.org/mod_auth_radius/

If you have access to Jonathan Hassell's RADIUS book it explains the module configuration well in Chapter 7. Let me know if you want me to type up a quick summary if you don't have access to the book.