You can write a .dll using C or other language firstly, then you call
it in Java by JNI.

You don't mention your operating system (keep in mind that the mere
notion of "process" is OS dependent - e.g. in some embedded
environments all you've got is coroutines).

As a quick and easy way, I'd try running a command-line utility first -
it's easier than doing JNI and gets the job done. You can use ps on
linux and use the one that comes with cygwin on windows. Also, the guys
at http://jniwrapper.com provide a nice windows integration library
that exposes much more than processes.

Under Windows XP you could use Runtime.exec() with the tasklist command
then parse the result to get the information needed.