Windows 7 or Windows Server 2008 R2 may restart unexpectedly, you may get a brief glimpse of a BSOD (Blue Screen of Death) and then find yourself back at the Windows login prompt.  When you inspect the Windows Event Log you will find the following event logged

Log Name: System
Source: Microsoft-Windows-Kernel-Power
Event ID: 41
Level: Critical
Description:
The system has rebooted without cleanly shutting down first. This error could be caused if the system stopped responding, crashed, or lost power unexpectedly.

The additional info under the event is important because the various bug check codes can indicate different causes.

EventData
BugcheckCode          0
BugcheckParameter1    0x0
BugcheckParameter2    0x0
BugcheckParameter3    0x0
BugcheckParameter4    0x0

Having faced this error recently it eventually turned out that the problem was due to incompatible memory (not faulty memory).  The batch of machines in question were all branded but due to an error with the manufacturers memory compatability matrix had been supplied with the wrong additional memory modules.  When we reverted the machines to the standard 2GB module that was already installed they operated perfectly, we eventually arranged for the additional modules to be swapped and that was the end of the issue.

An important thing to remember is that a memory checker will not show that your memory is faulty if it just incompatible with your particular system or mainboard.

Your system or mainboard manufacturers support should be able to confirm memory compatability and advise on the correct upgrade/type for your system