Compiling C-code from the Command Prompt in Windows?

You do this:

cl app.c

Here's a complete transcript, including setting up the environment for Visual Studio 2005 (change "8" to "9.0" for Visual Studio 2008).

C:\src\tests>"\Program Files (x86)\Microsoft Visual Studio 8\vc\bin\vcvars32.bat"
Setting environment for using Microsoft Visual Studio 2005 x86 tools.

C:\src\tests>type app.c
#include <stdio.h>
int main(void)
{
    printf("Hello world!\n");
    return 0;
}

C:\src\tests>cl app.c
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 14.00.50727.762 for 80x86

Copyright (C) Microsoft Corporation.  All rights reserved.

app.c
Microsoft (R) Incremental Linker Version 8.00.50727.762
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:app.exe
app.obj

C:\src\tests>app
Hello world!