What is a good C compiler for Windows Vista?
Mingw works. ( http://www.mingw.org/ ) Mingw is a port of GNUCC to Windows.
You can also use Code::Blocks IDE which comes with MinGW.
TinyC Compiler is another compiler, but i prefer MinGW myself. :)
dev C++ is a good and lightweight compiler for any version of windows. http://www.bloodshed.net/devcpp.html
If you're willing to dive into the command line (and as a programmer, you should expect to eventually), the Gnu Compiler Collection is a great choice. Its been around since pretty much forever (1980s in real-life terms) and it or its offshoots can compile most anything (Ada, anyone?) and target most anything (TIGCC if you want to write for your TI graphing calculator).
If the command line is still too scary, there are a few front ends to make things easier.
GCC, however, isn't a full on development environment. You'll have to use a separate text editor, of which there are many good ones on Vista (I'm partial to Vim for Windows, Emacs, and Notepad++, all of which support syntax highlighting and auto-indentation).
The free command line Borland C++ 5.5 is good.