There are, as you say, many ways to do this. What I do is simple:
1) I capture the screen using Alt + PrScr to copy the active window to the clipboard.
2) Open Fireworks and create a new file using Ctrl + N. Automatically it creates a canvas with the size of the image in the clipboard.
3) Paste the image using Ctrl + V.
4) Export the file using Ctrl + Shift + X
5) Select the tab "File" and there write the wanted width.
6) Press Export and write a filename.
That's it. Nothing fancy, just practical. If you want something else, visit a freeware or shareware repository and look for "screen capture". You'll find at least five or six good free programs.
Hope this is useful.