1. Go to Start > Run (or Windows-key + "R"), type in "regedit" and hit "OK".
2. Navigate to the key "HKEY_CURRENT_USER\Software\Classes\Local Settings\Software\Microsoft\Windows\CurrentVersion \TrayNotify"
3. Delete the values "IconStreams" and "PastIconsStream"
4. Now open up the Task Manager (Ctrl + Shift + Esc), go to the "Processes" tab, select "explorer.exe" and click "End Process"
5. Now open the "Applications" tab and click "New Task..." at the bottom-right of the window.
6. In the message box that pops up type in "explorer.exe" and hit "OK"
7. Explorer.exe will reload, and the missing icons should now be back in the system-tray where they belong.