It is very easy to resize pictures in Windows!
You can always right click on a picture and choose "edit" to open in Paint
(Alternative is to press "Start --> Accessories --> Paint, the open your file manually)
When in paint with the picture open, press 'Crtl and R' at the same time to bring up the resize box.
Then type 640 PIXELS (not percent) as the max width.
If you have the 'Maintane Aspect Ratio' box checked, the height/length wil follow automatically.
Save as new file if you are keeping the picture for yourself, or just save and overwrite if you don't care..
The picture is then ready for posting on forum
Hope this helps!
By the way, you can always check youtube for a tutorial on resizing images in paint by simply searching for 'resizing pic paint' or by clicking this link if you're lazy like me ^^
Enjoy!