Delphi7 does not support PNG format by default but there was a freeware PNG library for it. So I gues that previously you also had that library instaled but lost it due to reformatting.

NOTE: When formatting your computer it is always good to make backup of your Projects folder (of course), Delphi Lib folder (many components are installed into this folder) and Delphi source folder as some of hte components gets installed into this folder.