The windows help file for hotkeys:
Doesn't list the WindowsKey + B as an option, here is how it works:
1)Press and hold Winkey + B. This will shift focus to your systray.
2)Use arrow keys. This will navigate across the icons, showing tool tips for each icon.
3)Press the enter key activate them (simulates a double click).
4)Press menu key to bring up the menu for that icon.