I just got done with an extremely painful 3+ hour installation of Windows 7 SP1 on my laptop which has an ATI Radeon Mobility HD 5870 GPU. I received a 0x800f0826 error during the install, preventing me from successfully installing Windows 7 SP1. After a good amount of research and testing, I was able to locate a fix for the problem, and discovered that the original source was Driver Cleaner .NET. After every driver uninstall I perform my standard operation of running Driver Cleaner .NET with the latest Live Updates installed using the ATI cleanup option. Apparently during this process critical Windows system files (specifically ATI driver files) were deleted by Driver Cleaner .NET that the Windows 7 SP1 install requires to exist in order to successfully install the service pack. This issue also plagues the Driver Sweeper application (which I have never run) and there is additional info here: Windows 7 Service Pack 1 (Page 1) / Driver Sweeper / Phyxion.net Forums I had to copy the files in question from one of my other Windows 7 PCs, which allowed me to complete the Windows 7 SP1 install on my laptop. Do recent versions of the ATI cleanup option still delete the driver files out of the following locations? EDIT: Fixed the 3rd directory as my memory failed me. %windir%\winsxs %windir%\winsxs\Manifests %windir%\System32\DriverStore\FileRepository If so, those directories should be removed from the ATI cleanup option or a permanent exclusion should be added to prevent issues like the one I had from occuring in the future. I understand what Driver Cleaner .NET is trying to do and acknowledge that Backups would have allowed me to recover more easily, but those folders should probably stay off limits for stability reasons.