Hello friends

Been trying to update my MS OS for some time. The current version is 10.5.6 and need to update it to the next one.
Every time I try and do it, at the end of the massive 700 odd MB download, it tells me that signature is tampered or something and hence it can't be saved/ installed.
I haven't done anything funny with the OS etc hence difficult to figure the solution. Also I live in a place, where the broadband speed is very slow and takes ever to download.
Any solution? Can I get it copied on a disk? Not much on ground support available in Mumbai..

Thanks in advance