I deleted the Additions folder in Virtual Box.
 

I accidentally hit remove when I told Virtual Box to mount the image for additions in Windows XP. Now I don't know what to do. I thought I could just copy the additions folder from an old Virtual Box server I have running on another computer and paste it into the one that's missing it but I don't know where to paste it inside of the Virtual Box directory. Can anyone help me out?

If you mean you no longer have the additions.iso to mount, simply choose install additions and another will be downloaded for you.