It appears somone (or something) is trying to hack into our server.  Is our server safe?

I have been receiving this error message (see the snippet below) since the beginning of July.  When I look at the Security event log I see a FAILURE AUDIT. When I examine the details I see that different user names and passwords are being tried to gain access to the server.  So far, the username and password names have not matched and I don't believe that anyone has gained unauthorized access to the server.  The attempts last usually about 30 minutes, but some have been longer and there are typically 2 attempts per minute.  There is typically one attempt per day at differing times.

When I go to the System log I see warnings saying that the server was unable to logon the the Windows NT account "xxxxx" due to an unknown username or password.  The name XXXXX changes all of the time unless they ar trying to gain access using the administrator or variations of administrator account.  I assume they are using the administrator acccount with different passwords.  

Is there something that I should be doing to deflect these attempts to gain access to our server?
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Source Event ID Last Occurrence Total Occurrences 
  Security 529 7/21/2008 4:34 AM 8,449 * 
Logon Failure: 
  Reason: Unknown user name or bad password 
  User Name: julian 
  Domain: XX (I've changed our real domain name)
  Logon Type: 8 
  Logon Process: IIS 
  Authentication Package: MICROSOFT_AUTHENTICATION_PACKAGE_V1_0 
  Workstation Name: SERVERNAME (I've changed our real server name)
  Caller User Name: SERVERNAME$ (I've changed our real server name)
  Caller Domain: XX  (this was our own domain name - I've changed our real domain name)
  Caller Logon ID: (0x0,0x3E7) 
  Caller Process ID: 712 
  Transited Services: - 
  Source Network Address: - 
  Source Port: -