My system (WinXP Pro (SP3)), running on a Lenovo M55e desktop keeps once in a while self rebooting. The last 3 reboot occurred during 28 hours.
The only log report I can relate to these reboots is the following event log report:
"The Windows Security Center Service was unable to establish event queries with WMI to monitor third party AntiVirus and Firewall."
Source: Security Center
Event ID: 1802.