Event 529 Logon Failures - Many

My Windows SBS 2003 Server Security Log recorded (138) 529 logon failure events during a 15
minute interval. one failure about every 6-7 seconds.The user names were male
and female first names.

Is there a way to trace this to the source to find out who is doing this?
Is there a way to lock out the intruder?
I only forward ports for exchange server, OWA, RWW, and Terminal Server through my router to my sbs server. The login failure events don't have a port number associated with them. How did they get forwarded to my server??? A sample of one of the events follows.
Thanking you in advance for your help.

Security 529 2/20/2005 7:27 PM 24 *
Logon Failure:
Reason: Unknown user name or bad password
User Name: crack
Domain:
Logon Type: 3
Logon Process: Advapi
Authentication Package: MICROSOFT_AUTHENTICATION_PACKAGE_V1_0
Workstation Name: SERVER
Caller User Name: SERVER$
Caller Domain: domain
Caller Logon ID: (0x0,0x3E7)
Caller Process ID: 1828
Transited Services: -
Source Network Address: -
Source Port: - Start Free Trial