Daily restart

Hi,

My recently fully reinstalled updated PC running XP does restart by itself every night around midnight.
I found an event in application log right before the restart:

---
Event Type: Information
Event Source: ESENT
Event Category: Online Defragmentation
Event ID: 701
Date: 19.09.2008
Time: 0:00:04
User: N/A
Computer: [myPCname]
Description:
msnmsgr (3496) Online defragmentation has completed a full pass on database '\\.\C:\Documents and Settings\[myusername]\Local Settings\Application Data\Microsoft\Messenger\[mymsnaddress]\SharingMetadata\Working\database_7488_29ED_8829_AF14\dfsr.db'.

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

Same message every night right before restart. As this is related to msn messenger, i closed it before i left the pc. Restart happend now 1:48AM

I found another event in security event right before every restart:

---
Event Type: Success Audit
Event Source: Security
Event Category: System Event
Event ID: 515
Date: 20.09.2008
Time: 1:49:43
User: NT AUTHORITY\SYSTEM
Computer: [myPCname]
Description:
A trusted logon process has registered with the Local Security Authority. This logon process will be trusted to submit logon requests.

Logon Process Name: KSecDD

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

Any ideas, how to find out the reason for the restarts?

---
Jaanus

 

There is F-secure Anti-Virus - thanks for the idea - i'll try to reinstall it.