I had a similar problem a few months ago where I could download various language packs and minor updates, but couldn't do any security updates. My first thought was to do some malware scans and clean the registry but that didn't fix it. Finally I gave up and called MS tech; I watched the tech do everything I had been doing previously before she was like ok, I'm just gonna reinstall Windows. IMO that's always the last option, and I didn't lose any data but it took forever. Fixed the problem though.