Would it be better to transpose a Microsoft .NET program to standalone by modifying the existing code, or to just use the existing code as a reference? By standalone, I mean that the .NET framework (nor any other) would not have to be installed on the machine. Preferably, I'd prefer the option which would take the least time, even if it is slightly buggy.
What about using the Native Image Generator?
http://msdn.microsoft.com/en-us/library/6t9t5wcf(VS.80).aspx
If you mean "run without the .NET framework"... it isn't a good idea, and isn't really supported. But you can try the options that Jon lists here: Does the framework have to be installed to run my .NET application?
It depends how extensive the use of .NET is in the existing code. Some managed C++ looks very similar to native C++ -- in this case you'll be better off replacing the .NET parts with equivalent native C++ libraries. On the other hand, some managed C++ uses a lot of .NET-isms such as ^ references throughout the code, in which case a re-write is probably in order.
I know you mentioned not using any framework, but, for the sake of your sanity I'd suggest something like what used to be Borland's VCL (I haven't used it in a while so I'm not sure what it's called now). To not use a well-designed framework, when there are options available ... well, it's just hard not to call that a huge waste of resources. At least for a standard Windows program.