The upside of TextPad is that in addition to providing syntax highlighting, you can also compile and run programs from within it. Also, you can use it with other languages such as Java, Python and Perl.
The downside is that it costs $40.
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".NET Studio." - a open source editor for .NET is SharpDevelop. I'm guessing that becasue .NET supports C++, it should then support C. You may be able to write C code and complie with the .NET C++ tools. (Could be wrong here!)