This is a discussion on Desktop not centered. within the Windows XP Support forums, part of the Tech Support Forum category. I noticed that my wallpaper is not centered (even if it's set to Center) when my icons are hidden.
This may seem odd, but I had the same exact problem. Are you using the new IE8? I uninstalled IE8 and my wallpaper now stays centered when I right click on the desktop/arrange icons by/show desktop icons. I also noticed that IE8 corrupted my printer driver and I had to reload it's driver and relative software. I hope this helps.