This is actually quite a simple fix.

For those of you looking for a simple solution to this:

Fire up my computer; go to C:\Program Files\Steam\steamapps\

Remove winui.gcf

In the past I removed ClientRegistry.blob & friendsUI.dll from the C:\Program Files\Steam\ folder and that sufficed. However since the original posting, it seems that has changed.

If your still having trouble, post a comment.