it is in the xml file -
System > Language Definitions
Then in the field box Translation type in part of the text you are looking for - in this case Thank you for registering, click enter key
That will bring up all the places that text is used
click the edit on them until you get the ones you want, then change it, and Save
NOTE: there are ofter two sections - one for STOREFRONT (what your customer sees) and one for ADMIN (what you see)
be sure you are on the correct section to change it.