Website hacked

Hi,

Our blog was hacked somehow where the index.php files were modified (in root folder – an where it appears in any sub-folders). Our backup on the hosted site also seems to have been hacked so it looks like some auto procedure.

My question: is there a way to re-install our obtain copies of just the index.php files? And are the php files standard, or are they generally modified as part of an installation?

Thanls