Is this a virus, hacker or other program

I am having trouble finding the cause of this problem, I think it was also the cause of my computer restarting for no reason....

Computer reinstalled 2 days ago
All critical updates downloaded 2 days ago (there was a time while they were being downloaded that the computer has security holes)
I used AVG antivirus (Says no virus, but do not trust it)
Scanning with Nortain (still no virus)
Scanning with Ad-ware (no results so far)
Use sygate..AND..

Although there is only one user on this computer, there is strange logins. The one below happened only moments before the computer restarted...

Event Type: Success Audit
Event Source: Security
Event Category: Logon/Logoff
Event ID: 540
Date: 9/07/2004
Time: 1:12:45 a.m.
User: NT AUTHORITY\ANONYMOUS LOGON
Computer: BOB
Description:
Successful Network Logon:
User Name:
Domain:
Logon ID: (0x0,0xD404)
Logon Type: 3
Logon Process: NtLmSsp
Authentication Package: NTLM
Workstation Name:
Logon GUID: {00000000-0000-0000-0000-000000000000}

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