Event Viewer - Security

I am a Linux admin, but have the added responsibity of supporting Windows XP boxes somtimes.  So my windows knowledge is not great.  

Sometimes when I log into a windows XP machine I get a Security log is full message.  I then have to login as admin and clear the events before I am able to login as a regular user.  Most of the events are as follows:

Event Type:      Success Audit
Event Source:      Security
Event Category:      Logon/Logoff
Event ID:      538
Date:            7/9/2008
Time:            11:42:58 AM
User:            NT AUTHORITY\ANONYMOUS LOGON
Computer:      My-Win-Box
Description:
User Logoff:
       User Name:      ANONYMOUS LOGON
       Domain:            NT AUTHORITY
       Logon ID:            (0x0,0xD06F907)
       Logon Type:      3


For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.

Also this type:

Event Type:      Success Audit
Event Source:      Security
Event Category:      Logon/Logoff
Event ID:      540
Date:            7/9/2008
Time:            11:42:58 AM
User:            NT AUTHORITY\ANONYMOUS LOGON
Computer:      My-Win-Box
Description:
Successful Network Logon:
       User Name:      
       Domain:            
       Logon ID:            (0x0,0xD06F907)
       Logon Type:      3
       Logon Process:      NtLmSsp
       Authentication Package:      NTLM
       Workstation Name:      My-Cube
       Logon GUID:      {00000000-0000-0000-0000-000000000000}

For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.

I'm not sure how to interpret these?  Are these Anonymouse entries unusual?