I had problems with the new patch and i could not load the game, so I tried a bunch of things and reinstalled diablo 3 several times to fix it and I am using windows xp
1) Disable or turn off your firewall, sometimes even if disable the firewall still works. I disabled my Kaspersky firewall, but somehow this blocked my connection to the battlenet patch server still. (Found out I had to uninstall kaspersky firewall to fix my problem with installing diablo 3, patch up date and load the game after many hours.
2) Before you uninstall, run the repair tool on the launcher, see if it fix your problem.
3) Go into internet explorer, go tools, internet options, lan settings, check automatically detect settings and uncheck use automatic config scrip/use a proxy server. If above does not work, go advanced in tool options, reset internet explorer settings, sometimes spyware change your settings.
4) Go microsoft website and update your windows and internet explorer, found out battlenet update use internet explorer for patch download.
5) Go C:\Documents and Settings\All Users\Application Data or similar one in your windows, delete battlenet and blizzard entertainment folder (when you run launcher they reinstall these and make a backup copy in case)
6) Go Start menu, run, type in msconfig for windows xp or similar one in windows 7/8, go general, set selective startup, disable all items in start up. Go services, hide all microsoft services, disable all others (remember your settings here change back to it later) Restart your windows. Try install diablo 3 or patch up date with launcher again, and try repair tool in launcher.
7) If none of the above works, uninstall your firewall and antivirus. Restart computer, try install diablo 3 or patch up date with launcher again.