PowerPoint is a great tool for creating simple (and not so simple) diagrams. Copy and paste of course works with the other Office apps, and many other programs understand the PowerPoint format. It is also possible to export any or all of your slides (and so diagrams) in a number of different image formats. Glancing down the list in the ‘Save as’ dialog reveals options of GIF, JPEG, PNG, TIF, BMP.
However, saving a diagram in an image format for a presentation or as a small illustration in a Word document is one thing. Being able to use that image in a high quality document is another. The default resolution is 96 dpi (dots per inch of your diagram on the slide) – a strange number, whose history is tangled up with the original screen resolution of the Mac, so I’m told.
For high quality images, a resolution of at least 200 dpi is needed. Sometimes higher.
I was recently pointed at a Microsoft support article, that describes how to increase the output resolution (to a maximum of about 300 dpi).
WARNING: to change the resolution you need to make a change to the Windows registry. There is potential for disaster there, so make sure you follow the instructions very carefully – including the instruction to make a backup copy of the registry!