Im using a custom theme i created. I created a folder, /wp-content/themes/mytheme and created new versions of various php and css files to suit my need.
I dont remember changing any core files, or files outside of that folder, though i have installed some plug-ins.
I host the machine myself, and no backups are done on it, though they wouldnt help as I still dont know what file to look at for the code segment i need, and i can always get it from the install files which i have handy.
Olly