Random Reboot!? Help Please

My computer has been randomly shutting down without a crash log. Using event viewer I cant find anything that is causing this. This got so serious that it corrupted C:/Windows/Config so I had to reinstall my OS. After doing this and installing SP2 everything seemed fine, but now 2weeks on, it is doing it again.

Strangely though when the computer is shutting down, the computer tries to open "Calc.exe" but can't because the computer is shutting down.

I thought that it might be a virus or a trojan but Norton AV doesn't pick up anything unusual.

My Specs -
Windows XP Home w/ SP2
P4 3.06 GHz w/ HT (Northwood 533 FSB)
1028 MB DDR
ATi 9800Pro 128MBram
120GB hard drive
400 Watt PSU

 

I have got this unticked and I don't get any bluescreen. But it doesn't crash, it just shuts itself down.

These are the last three entries in event viewer beofre it shuts down.

Event Type: Success Audit
Event Source: Security
Event Category: Logon/Logoff
Event ID: 528
Date: 12/09/2004
Time: 07:40:29
User: NT AUTHORITY\NETWORK SERVICE
Computer: DOWNSTAIRS
Description:
Successful Logon:
User Name: NETWORK SERVICE
Domain: NT AUTHORITY
Logon ID: (0x0,0x3E4)
Logon Type: 5
Logon Process: Advapi
Authentication Package: Negotiate
Workstation Name:
Logon GUID: {00000000-0000-0000-0000-000000000000}

Event Type: Success Audit
Event Source: Security
Event Category: Privilege Use
Event ID: 576
Date: 12/09/2004
Time: 07:40:29
User: NT AUTHORITY\NETWORK SERVICE
Computer: DOWNSTAIRS
Description:
Special privileges assigned to new logon:
User Name: NETWORK SERVICE
Domain: NT AUTHORITY
Logon ID: (0x0,0x3E4)
Privileges: SeAuditPrivilege
SeAssignPrimaryTokenPrivilege
SeChangeNotifyPrivilege

Event Type: Success Audit
Event Source: Security
Event Category: System Event
Event ID: 513
Date: 12/09/2004
Time: 07:41:37
User: NT AUTHORITY\SYSTEM
Computer: DOWNSTAIRS
Description:
Windows is shutting down. All logon sessions will be terminated by this shutdown.

 

do a quick virus check on norton web page (google- norton) and also try looking for adware with a remover program and look in task manager under the tab processors for iosdt.exe
tell me the results plz