As Brian indicated above, the snipping tool works great for this. It's actually better than the print-screen or alt-print-screen options because it allows you to select just apportion of the screen.
As also mentioned, the image needs to be saved online somewhere. You can't just copy paste it into the forum, nor can you link to it on your local computer. It has to be stored online someplace. If you have a website, you could store it in a special folder there. If you have a Skydrive account (free) or similar, you could store them there as well. Each of these services will have their own way of allowing you to link to the stored images - usually via an embed code of some sort.
I'd prefer to keep them on my own website hosting location, as you can simple link over to the saved jpg and it just works. It's a little more techy complicated for some since you need to have the web hosting and understand FTP - not that it's difficult, but it seems to be a barrier for a lot of not-so-tech-savvy folks for some reason.
If you want to use Skydrive, you just save your image to the Skydrive location either via upload or by saving to the special location on your local computer that auto-uploads it. I have a Cakewalk folder there for mine. Then go to your skydrive account online, select the pic you want to link to and click Embed at the top of the screen, which will give you a line of code starting with <iframe. In order to paste this into the forum, you need to click the HTML button in the editor (only in the full version of the editor) and paste the iframe code there. Then you image will show up.
Does that help?