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In the root of the ipb "uploads" folder you have files like edituser.php, userinfo.php, user.php etc. You need to overwrite all these files on your server with your original XOOPS files to the root of your XOOPS installation.
Also make sure all the files in the ipb "class" "include" and "kernal" folders are restored to the original, you will need to look at each individual file that was used in these folders. Also check out the files in the ipb modules/system folder that need to be replaced and update your "system" files in system admin/modules.
Edit:
A good rule of thumb is never overwrite core files without first re-naming the original files on the server to something like: orgfile.php
Otherwise you can get into a real mess later when changing or upgrading..
