After you download the upgrade, disable your anti-virus (i usually unplug from the web at this point) and close down any other applications (this means everything! no mail, browsers, other games, windows update or anything else). The update, just as the original install, will use a lot of resources! It will take a very long time to install so be extremely patient and do not touch the machine until you see a message saying the update has finished!
The update will seem to pause or stall for an extremely long time. It is building the database and if you disturb it during the process, you will encounter problems!
