Failed to open the explicitly specified database error

Hi,
I've just installed a new SQL Server 2008 on Windows Server 2008 x64 (and SQL 2008 SP1), while it seems to be running well, I keep getting the same error appearing in the event log:

Login failed for user 'domain\username'. Reason: Failed to open the explicitly specified database. [CLIENT: x.x.x.x]

The Admin user name comes up once every 20 seconds or so, and the NT AUTHORITY\SYSTEM account appears twice every 5-6 seconds, obviously this is filling the event log pretty quickly...

The SQL Server event log has:

Error: 18456, Severity: 14, State: 38.

Then the above login failed message, the Windows event log has:

- System 

  - Provider 

   [ Name]  MSSQLSERVER 
 
  - EventID 18456 

   [ Qualifiers]  49152 
 
   Level 0 
 
   Task 4 
 
   Keywords 0x90000000000000 
 
  - TimeCreated 

   [ SystemTime]  2009-05-07T15:53:26.000Z 
 
   EventRecordID 20656 
 
   Channel Application 
 
   Computer servername 
 
  - Security 

   [ UserID]  S-1-5-18 
 

- EventData 

   NT AUTHORITY\SYSTEM 
   Reason: Failed to open the explicitly specified database. 
   [CLIENT: x.x.x.x] 
   184800000E000000080000004B004100420030003100530050000000070000006D00610073007400650072000000 


--------------------------------------------------------------------------------

Binary data:


In Words

0000: 00004818 0000000E 00000008 0041004B 
0008: 00300042 00530031 00000050 00000007 
0010: 0061006D 00740073 00720065 0000 


In Bytes

0000: 18 48 00 00 0E 00 00 00   .H......
0008: 08 00 00 00 4B 00 41 00   ....S.V.
0010: 42 00 30 00 31 00 53 00   R.N.A.M.
0018: 50 00 00 00 07 00 00 00   E.......
0020: 6D 00 61 00 73 00 74 00   m.a.s.t.
0028: 65 00 72 00 00 00         e.r...

I'm fairly sure that NT AUTHORITY\SYSTEM needs some sort of access to the master database, but I've tried adding that already and it didn't work.

Thanks,
Matt