Does anyone know? When I say PC I mean Windows, however, I would like to know the percentage of Macs that also have the Java Runtime Environment. I was considering what the viability was of creating desktop applications using Java as opposed to C++.

I think the percentage of Macs with a JRE is 100% -- it comes pre-installed with OS X, I believe. As for Windows, I'd wager probably a lot (definitely more than half, probably around three-quarters or more).

As for the viability of writing a desktop application in C++ or Java, there are a lot more computers with a JRE than there are with a C++ compiler and an operator who knows how to use it. That's one of the benefits of Java. It's all too unfortunate that Perl / Python / Ruby haven't caught on to that point of market penetration yet, but things may be heading in that direction.

i am confused by this because you dont need a c/c++ compiler to run a program.

i would assume that java is a lot less popular than 5 years ago. because grandma used to need it to browse the internet but now webpages are much more dependant on flash.

Yes, that's true; I mistakenly assumed that it would be necessary to compile the program from source in the case of e.g. multiple architectures or varying environments. Of course, that's not necessarily the case, esp. since he was inquiring specifically about Windows & OS X.

That is perfectly correct. You do not need anything to run native code.
Java is indeed less popular in web pages, but far more popular on the server side, and possibly on standalone application side too.