Installing Windows 7 Service Pack 1
1. Visit the PC manufacturer's website and update ALL
drivers. DO NOT
use Windows Update or the "Update drivers" function of Device Manager.
2. Check Device Manager for any unknown/disabled devices - if there are unknown/disabled devices, fix them with the latest drivers from the device manufacturer's website (not the PC Manufacturer)
3. Visit Windows Update and get all updates (may take repeated visits)
4. Visit Windows Update and get Service Pack 1 (usually under Important Updates). Read these notes for installing SP1: Steps to follow before you install Windows 7 SP1 and Windows Server 2008 R2 SP1 from the Microsoft Download Center
5. Visit Windows Update and get any other available updates. May take repeated visits, but keep it up until you get several "Windows is updated" results.
If you're having difficulties with installing a Service Pack, please use the SURTool from this link: What is the System Update Readiness Tool?
Also, check out this troubleshooting link from Microsoft: Troubleshoot problems installing Service Pack 1 (SP1) for Windows 7 and Windows Server 2008 R2
I have had limited success in installing updates/Service Packs when using a "clean boot". I suspect that I'm just bypassing the problem - and that it can come back to bite me in the butt later on.