If I was you I would check your "Windows Module Installer" in services. I also have had that problem for a long time, till I decided to do something about it today. If you can't install Service Pack 1 and also if you're having problems with installing iTunes, Windows Module Installer, may be the culprit.
First you will want to check and see if Windows Module Installer is working or if it's missing.
Click Start and in the search box type in "Services" then click on the one with the icon Shaped like a gear.
Scroll all the way down till you see "Windows Module Installer"
Once you find it, double click on it, you will see the preference menu pop up.
If your WMI (Windows Module Installer) Has stopped, click on Start, then in the middle you will see "Startup Type" Change that to "Automatic" and click "Apply".
That's how I fixed my Service Pack 1 issue today, after 5 months of not being able to install it.
Let me know if that helps you. If you can't find Windows Module Installer; type in google "Missing Windows Module Installer" or Open a Command Prompt.
Do a dir \Windows\TrustedInstaller.exe /s
See what directories TrustedInstaller.exe is found in. There should be one in C:\Windows\servicing. If not, copy it from one of the other locations it's in.
On my Win 7 Pro X64 system. it's in
Reboot, then start Services and see if the Windows Module Installer is listed. If yes, set it to Automatic and Start it.
I just copied and pasted that part from another forum, so I can't take any credit.