"If you try to install or uninstall Windows 7 SP1 Beta or Windows Server 2008 R2 SP1 Beta with Microsoft Security Essentials or Microsoft Forefront Client Security already installed on your computer, your security program might prevent the successful installation or removal of the service pack," Microsoft revealed.
To avoid this issue: "If you've Microsoft Security Essentials installed, install latest update to resolve the problem. You can then install or uninstall the service pack. Or if you've Microsoft Forefront Client Security installed, uninstall it before you install or uninstall the service pack, and then reinstall Microsoft Forefront Client Security."
More Info: Microsoft Support