Visit
http://www.reddit.com/r/GrandTheftA...und_a_fix_for_the_social_club_download_reset/ for a full description of what to do.
Summary:
Download MD5 checker via:
http://www.winmd5.com/
Open the software and in turn check each of the RPF files and see if the checksum value matches that of the list provided in the above Reddit link. If it does not match (mine was the x64e.rpf file) then delete it. The MD5 software has a verify option where you can paste the relevant checksum value for that file (provided in the Reddit thread) to make life a lot easier.
Load up the GTA V installer link again and it will re-download the missing file(s).
And another tip: If you do have to re-download anything again via the Rockstar installer and it finishes up and prompts you to retry, close the software and re-open it; everything was okay afterwards and I could finally play. I made the mistake of clicking retry after the 300MB patch and I think this is what messed it up. The installer re-downloaded the entire 358MB again.