Using Windows Update, let it update your computer with everything it suggests
apart from the optional stuff. Then assuming you have the full SP1 downloaded
switch OFF all your anti virus software and anti spy/malware programs, and dis connect from the internet. Once that's done, then click on the SP1 file and leave tne computer completely alone whilst it's updating. It may re-start a few times, let it. Finally the message will be displayed "Finished" or whatever. It's a slow process and can take about an hour, or more on slower machines.