Here are the shortcuts I use over and over again on Windows.

WindowsKey-e – Open Explorer
WindowsKey-l – Lock screen
WindowsKey-r – Open Run dialog
WindowsKey-m – Minimize all windows
WindowsKey-Shift-M – Restore all windows
WindowsKey-d – Toggle between showing the desktop and showing all the windows
WindowsKey-Pause – Show System Properties window (The same window you see when you right click and view properties on My Computer)
Ctrl-Shift-Escape – Launch Task Manager