JSI Tip 0035 - Managing the Mapped Network Drive dropdown list.

If you want to remove some the connections in the list, edit:

HKEY_CURRENT_USERSoftwareMicrosoftWindowsNTCurrentVersionNetworkPersistentConnections

Highlight and delete unwanted entires. Then double click Order and remove the letters that have been deleted. You may rearrange the letters to change the display order.