What worked for me was adding MinGW's bin directory to the PATH variable then restarting Eclipse. I then added this bin directory path to C/C++ | XL C/C++ Compiler under Window | Preferences.

1. Install MinGW.
   
2. Ensure Eclipse is NOT currently running, then insert the MinGW bin directory path (e.g. C:\MinGW\bin) at the start of your Windows PATH environment variable. I have XP, so I don't know how this is done in Windows 7. NOTE: I haven't specified a PATH variable under Window | Preferences | C/C++ | Build | Environment; I don't believe this is necessary in order for Eclipse to recognise MinGW as an available toolchain.
   
3. Run Eclipse, then in Window | Preferences | C/C++ | XL C/C++ Compiler, set 'Compiler Root Path' to the bin directory of your MinGW installation.
   
4. Create a new project using File | New | C++ Project (or C Project, etc.), then select one of the project type options. MinGW should be listed as one of the available toolchains for various project types.
   
5. Select MinGW as the toolchain to use for your new project, type in the project name, click Finish and then it will be created.
   
6. Under Project Properties | C/C++ General | Paths and Symbols, the 'Includes' tab should automatically be populated with the MinGW include directories, giving you access to all the standard C/C++ include files.
   
7. Under Project Properties | C/C++ General | Paths and Symbols | Library Paths, add the path of the MinGW lib directory (e.g. C:\MinGW\lib).
   
8. When linking occurs during a build, the standard MinGW libraries should be linked automatically. If you need to specify additional ones, put them in Project Properties | C/C++ General | Paths and Symbols | Libraries. An example: while developing a Qt application, I added QtCored4 and QtGuid4 to the list of libraries, as these contain the Qt graphics library functions I (currently) need. NOTE: You will need to specify different libraries for your Debug vs. Release configurations, e.g. the Qt libraries I mentioned contain the debug symbols, but for the Release configuration, I've specified QtCore4 and QtGui4 (note the absence of the 'd'). Interestingly enough, you can specify full pathnames and filenames in the library list, or just the library filename stubs that I mentioned above (and which appear in the console window in the linking command-line).