Today I finally got around to delving back into some WPF development only to be greeted with the following error when opening Visual Studio 2008.

Package 'Microsoft.VisualStudio.Xaml' has failed to load properly ( GUID = {E58C2A8B-BCC4-4559-AD59-D62EB6D58A22} ). Please contact package vendor for assistance. Application restart is recommended, due to possible environment corruption. Would you like to disable loading this package in the future? You may use 'devenv /resetskippkgs' to re-enable package loading.

At first I tried resetting my environment settings to no avail. Then I uninstalled/reinstalled VS 2008 with no love there either. Then I remembered that I hadn't yet installed VS 2008 SP1 so I did that and problem solved. So if you are receiving this error you may want to install/reinstall SP1 first and see if it solves your problem.