is to wait until the screen saver (especially if it's an animated one) shows the screen you wish to save, then press the Print Screen key on the keyboard. That sends just that particular screen to the Clipboard.
Now close the screensaver, open MSPAINT and click Edit, Paste and it will appear there. Now save the picture in JPG format instead of BMP as it will default to that, and once the file is saved, you can open it in any graphics editor and do what you want with it.
I had a hard time figuring out how to do this long ago when I had a screensaver that was created in fractuals, (kind of like a kaleidoscope that constantly swirled and changed) and a ZDNET (CNET bought them out) member did it for me after I sent him the screensaver file. When he told me how he did it, I was amazed at how simple it was.