On Any Version of Windows since Win95
1) Hit the Print Screen key, nothing appears to happen, but an image of the desktop has just been copied to the clipboard.
2) Open up any graphics editor. MS paint is built in, and will do. But so will a couple dozen more.
3) Paste the image (ctrl-v). Crop and save.
Create a new thread (or reply to an old one)
Type SOME text in the body of the message. At least one word.
Click on the link that reads "attach a file", It will bring up a button that reads "browse".
Click on the Browse button. This will bring up a dialog that will allow you to select an file.
Find your file, click "open"
When you are satisfied with your message, click on "post"