I use a free program called IrfanView. It resizes and does a bunch of other things, but I use it for bulk resizing. If you always want the same output resolution, you can set the parameters the first time and they stay set unless/until you change them.
A friend use Faststone, another free program.
Lastly, for just an occasional resize, the Windows 7 (and newer) version of Microsoft Paint allows you to resize by either percentage or pixels. (The XP version of Paint also resizes, but it only does percentage so that means either trial and error or using a calculator to figure out what percentage of reduction you need.)