there are 3 other inexact ways in XP, only 1 requires a download.
You can open the jpg in paint (programs/accessories/paint - or right-click and choose 'open with' and select paint) and change the image attributes (image menu, attributes) by reducing the number of pixels. Save your image and check size. repeat if necessary.
Alternatively, if you put the image file on your desktop (or somewhere else easy to access) right-click and choose send to - mail recipient. This should bring up an small box offering to reduce the size of your image. Choose the smallest option. A new email will come up with your photo attached. If it is now small enough you can either right click from there and choose 'save as' and put it somewhere or you can copy and paste it somewhere. The results on this method are a little random but it usually shrinks it quite a bit.
If the powertoy is not your go (I quite like it and miss it in windows 7
) then you can download Irfanview which is a free image manager that will give you lots of control over stuff like size etc. This won't be an option if your IT people have locked you down on a work computer or whatever.