Well, others have already told you haow to take the screen capture using the "ALT + print screen" key combo. As for the paint program I recommend .
Irfanview. It's more of an Image viewing program, but it will allow you to save the image to a wide variety of Image formats. It's also free which is always a nice bonus.
A nice tip is to convert the Image to a 16 color GIF file. Notice, I said 16 color, not 16bit color. This will make it smaller and more modem friendly. If you think about it most of these benchmarks programs display their results in a widow that only uses a few colors. So, there's really no need to use 24bit color when your image only contains 3 or 4 colors. If it looks too bad with only 16 colors then try using 256 colors. 256 colors can actually be pretty good sometimes because it's palletized. Anyway, 256 colors will be perfectly sufficent just for posting screen caps of benchmark results and the page you post them on will load alot faster for those without a highspeed connection.