As you use XP you already have the software.
Open Paint - Start:Accessories:Paint
Open in Paint. The image will be larger than the screen.
Now go to Image:Stretch/Skew. Under horizontal change the value from 100 on vertical and horizontal to 75. This will reduce the image size by 50%. You can then save with the new size and replace the original or better save with a new name and keep the original.
If you want it smaller again repeat the step - 80 80 will reduce by about 36% and so on.
If you download Picassa from Google this will also give you some rudimentary editing and also enable to you resize on the fly when you send the images. In this case the size will be in the range 30-60k but the original will remain untouched. Picassa is free.
PN