Does the Task Parallel Library (or PLINQ) take other processes into account?

In particular, I'm looking at using TPL to start (and wait for) external processes. Does the TPL look at total machine load (both CPU and I/O) before deciding to start another task (hence -- in my case -- another external process)?

For example:

I've got about 100 media files that need to be encoded or transcoded (e.g. from WAV to FLAC or from FLAC to MP3). The encoding is done by launching an external process (e.g. FLAC.EXE or LAME.EXE). Each file takes about 30 seconds. Each process is mostly CPU-bound, but there's some I/O in there. I've got 4 cores, so the worst case (transcoding by piping the decoder into the encoder) still only uses 2 cores. I'd like to do something like:

Parallel.ForEach(sourceFiles,
    sourceFile =>
        TranscodeUsingPipedExternalProcesses(sourceFile));

Will this kick off 100 tasks (and hence 200 external processes competing for the CPU)? Or will it see that the CPU's busy and only do 2-3 at a time?