Luckily there are a number of tools you can use for speedily batch processing your image files. Image Resizer is a clone of the original Image Resizer Powertoy build for Windows XP, which has been created to extend its support to non-XP and 64-bit versions of Windows, including Windows Vista and Windows 7.
Using this free and open source utility couldn't be simpler: just right-click one or more image files, select "Resize Pictures" from the context menu, choose a preset resolution and you are ready to go. You can also custom resize images by clicking the "Advanced" button and setting the parameters, or chose "Resize the original pictures" if you would like to overwrite them instead of making copies.
Image Resizer does its job remarkably well and characteristically simple but is rather limited in the features department -- you won't be able to convert between different graphic formats, for example, or bulk-rename files. If you are looking for something a bit more advanced, then VSO Image Resizer might be worth a look.