For today, two ways to pixelate images on Windows. One is insanely easy. The other is insanely handy.

First, the handy method. From instructables.com:

Use the rectangular Select tool to select the area you wish to pixellate.
...
From the Image menu, choose Stretch/Skew... (CTRL-W)

Reduce the size of the area both horizontally and vertically. Here I am making the area 10 x smaller by selecting 10%. 
...
From the Image menu, [now] choose Stretch/Skew... (CTRL-W) again.

Paint can't stretch selections more than 500% (5 x) so to increase the selection back to it's original size I need to stretch by 500%, then repeat the stretch at 200%.

That's clever. And you can do it anywhere you have mspaint.exe, so pretty useful too. Kinda disappointed I never thought of that. Shrink and increase back, and poof, you've performed a lossy/pixelating operation, duh.

Now I'd been using Skitch's pixelation tool when making screenshots of apps/systems with sensitive info, but didn't realize I could skip a step from my normal "alt-PrtScn, paste into IrfanView, cut just what you want, paste into Skitch, pixelate, and save" workflow if I didn't also need to annotate.

Turns out IrfanView can pixelate for you. The results are potentially not as pretty as some other options, but it is handy.

If you haven't used IrfanView and you do anything with images on your Windows boxen, you should take a look.