Look in Event Viewer for an error message. OR Turn of auto rebooting (then you'll see the reason). Post exact text of whichever. Turn on mini dumps (it may be required for further diagnosis).
From Help
To specify what Windows does if the system stops unexpectedly.
You must be logged on as an administrator or a member of the Administrators group in order to complete this procedure. If your computer is connected to a network, network policy settings might also prevent you from completing this procedure.
1.. Open System in Control Panel.
2.. On the Advanced tab, under Startup and Recovery, click Settings.
3.. Under System Failure, select the check boxes that correspond to the actions you want Windows to perform if a Stop error occurs:
a.. Write an event to the system log specifies that event information will be recorded in the system log.
b.. Send an administrative alert specifies that your system administrator will be notified.
c.. Automatically reboot specifies that Windows will automatically restart your computer.
4.. Under Write Debugging Information, choose the type of information you want Windows to record when the system stops unexpectedly:
a.. Small Memory Dump records the smallest amount of information that will help identify the problem. This option requires a paging file of at least 2 MB on the boot volume of your computer and specifies that Windows will create a new file each time the system stops unexpectedly. A history of these files is stored in the directory listed under Small Dump Directory.
b.. Kernel Memory Dump records only kernel memory, which speeds up the process of recording information in a log when the system stops unexpectedly. Depending on the amount of RAM in your computer, you must have 50 MB to 800 MB available for the paging file on the boot volume. The file is stored in the directory listed under Dump File.
c.. Complete Memory Dump records the entire contents of system memory when the system stops unexpectedly. If you choose this option you must have a paging file on the boot volume large enough to hold all of the physical RAM plus one megabyte (MB). The file is stored in the directory listed under Dump File.
Notes
a.. To open System, click Start, click Control Panel, and then double-click System.
b.. You must have at least a 2-MB paging file on the computer's boot volume if you select Write an event to the system log or Send an administrative alert.
c.. If you choose either Kernel Memory Dump or Complete Memory Dump and select the Overwrite any existing file check box, Windows always writes to the same file name. To save individual dump files, clear the Overwrite any existing file check box and change the file name after each Stop error.
d.. You can save some memory if you clear the Write an event to the system log and Send an administrative alert check boxes. The memory saved depends on the computer, but typically about 60 KB to 70 KB are required by these features.
e.. If you contact Microsoft Product Support Services about a Stop error, they might ask for the system-memory dump file generated by the Write Debugging Information option.
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