Context menu handlers
One of the reason Windows systems seem to get slower over time is because Microsoft has provided so many places to 'hook into' Windows, and so many developers have done a bad job of doing this.
My wife's computer was taking a long time - over a minute - to bring up a context menu when she'd right-click on a file in the Explorer. Using this tool, I discovered that it was the RealPlayer context menu handler that was taking all the time. Disabled it, and the context menu is snappy again.