1) Use the Windows Snipping Tool to grab a chunk of the screen. It's been in Windows since Vista, so unless you're running XP (and you really, really, _really_ shouldn't be) you have it.

2) Open
Imgur.com3) Click "Upload Images":

4) Press Ctrl-V:

5) Click Start Upload:

6) Click on the icon next to the Direct Link:

7) Make an image link using it: <img src="
http://i.imgur.com/k9lJ5Zf.png"> (I use Semagic to write posts, so I simply paste the link, select it, and press Ctrl-Alt-I to turn it into an image link).
I was using Photobucket at one point, but imgur makes it so easy to upload screenshots and suchlike that I've pretty much entirely moved over to using it.