Any photo editing software will do. Even the most basic microsoft paint will allow you to crop and resize. I use Photoshop, Paintshop Pro, and Animation Shop for different things. Heck, I even used the free Photobucket editor to do some cool stuff.
If your new to photo editing, there is a whole lot more you can do with these programs.
For simple resizes I use the microsoft resize powertoy.
http://www.microsoft.com/windowsxp/Downloads/powertoys/Xppowertoys.mspxI can right click any file on my computer and resize it without opening any programs.