For Windows XP, Microsoft has an easy to use image resizing Powertoy which you could download at Microsoft's website .
You just right-clicked a jpg file in Windows Explorer and could easily re-size it.
The problem is that this tool is not available for Vista.
Most people can only complain, but there are others who can do something about it. And someone did. There is an Image Resizer Clone available for Vista (and Windows 7). You can download it at http://www.codeplex.com/PhotoToysClone.
The version at the time of this writing works great. Only things to watch for - make sure you download the right version (32 or 64 bit) and be aware that it strips out any colour profile that may have been saved in the image.
Finally a quick and easy resize option again.