Last year when it was released, I installed the free Image Resizer tool which part of the Microsoft PowerToys for Windows XP. It only takes a few seconds to install, and there is no icon shortcut created. Instead it is added to your system right-click context menu for image files in Windows Explorer. Now when I take pictures using my digital camera and load them into a folder, I can select one or all of the JPG files and right-click, then select the Resize Pictures menu item to get the Image Resizer dialog. You can then select any size, from a list of pre-selected ones like 640x480 or any custom size, plus other options too. It will automatically create files with the same name but with a (Small) or (Custom) part in the file name before the file extension, like DSC00123 (Custom).jpg. What I do is create a subfolder in directory of my JPG photos called Resize, then yet another subfolder like Small or the name of what I want to use those resized images for, then cut and paste the new resized files into that folder.
 
I generally take a bunch of photos on a trip in one day, and each are over 3MB using my 8 megapixel Sony F-828 digital camera. Then I take the memory card (Compact Flash or Memory Stick, the F828 has both slots) and place the card into a tiny card reader and connect it to my USB 2.0 port on my notebook computer. I then copy the files over to my hard drive, then format the memory card for maximum memory available the next time out. Then I select all the new big JPG files by using the Resize Pictures PowerToy to create a set of smaller ones like 640x480, zip them up and email them to friends and/or family. If you use a portable device like a PocketPC, Smartphone, or something else like the new Sony PSP (I have all 3 of these gadgets), you can resize the images to the exact size that the portable unit displays. This will save memory space on the memory card and also will  increase the performance of scrolling between pictures and slideshows since smaller files take less time to load and the right pixel size photo for the mobile device doesn't have to resize the image.
 
There are many other free PowerToys available too. The other download I sometimes use is the Power Calculator instead of the built-in Windows calculator since it you can evaluate functions as well as perform many different types of conversions, plus it has a history list.