But if you want to continue to use the January CTP of EID then please read this post before rushing off and installing anything.
As you know, the WinFX Runtime Components (RTC) are the redistributables (runtime binaries) needed for executing WinFX applications. You target a particular version of WinFX when you develop your Windows Presentation Foundation (WPF) applications. You’ll also be aware that Expression Interactive Designer (EID) is itself a WPF application so it is built to run against a particular version of WinFX, as are the WPF applications you build using EID.
So, if you want to continue to use the January CTP of EID then you’ll need the WinFX Jan CTP installed on your machine. If you’ve downloaded and played with the new WinFX Feb CTP then you’ll need to revert to the WinFX RTC Jan CTP  in order to continue to use EID.
That is, at least for just a little while longer. We have a March CTP of Expression Interactive Designer under starter’s orders as we speak so please be patient just a little longer. And the EID March CTP will work with the WinFX Feb CTP.
If you’d like to experiment with the new WinFX RTC Feb CTP  then please do so. If you’re a developer-type then you’ll also be keen to try out the compatible Visual Studio “Orcas” CTP  which contains a WPF visual designer (code name “Cider”).