It's not possible to emphasise enough the importance of using sensible passwords on your network.
Not just on the areas of your network that you don't want your users to traipse through, but also on the default network shares that are present on installations of commonly used operating systems like Windows NT/2000/XP/2003.
One of the ways in which the Conficker worm (also known as Confick or Downadup) uses to spread is to try and batter its way into ADMIN$ shares using a long list of different passwords.
As you can see in the list below, it relies upon computers using poorly chosen passwords such as dictionary words, "password", "qwerty" or sequences of letters or repeated numbers:
One way to make it harder for password-cracking malware like Conficker from spreading across your network is to ensure that no-one is using a poorly-chosen password.
And, of course, please don't delay installing the critical security patch that Microsoft issued late last year.