here is link to correct driver package, make sure it is rtl8187L

Realtek

Choose the auto install option(make sure you remove all realtek drivers for utility first from add/rem programs. (for os vista, xp, etc this is item to download)

If you run windows update manually, you willl see an option to 'upgrade hardware' (other updates) for your alfa adapter, this will give you the latest driver for this item.