problem with update.inf and cryptographic service error help please???

im trying to download windows xp sp2 from the internet.Does extracting files etc. fine,but it wont continue the rest because im missing update.inf and cryptographic service...says i have a cryptographic service error and it needs to be fixed before i can do the rest.Can someone out there please help me figure this out ,or how to fix this problem or even download.Id appreciate any help or advice yall can give me.Thanks