<p>I am building Windows apps for a few clients.  I read Joel on Software and took the Joel Test and realized I don't quite measure up.  One place I am lacking is automated builds.  What should I use to have automated builds?  I have Windows apps that use .NET 1.1, 2.0, and 3.5.  Also, I need to be able to build my .vdproj to create MSI's.  I am looking for something that is free and would work well for a one-man team.</p>

CruiseControl.Net
and
NAnt
are the standard tools (and open source too), so there will be a lot of community support available.
MSBuild
is now open source, too
It will be quite a lot of work to set it all up, but it will be very powerful and have lots of room for growth too.