Reboots of this nature can be just about anything, Software/OS, drivers, faulty memory, hard disk motherboard even the PSU.
Memory is fairly easy to check with memtest, hard disk checked with manufactures extended diags (though they tend to be destructive). I have had to date two issues where customers complained of these sorts of symptoms and running the quick hard disk tests came back as OK. Running the extended diags on these disks returned error codes at about the 99% complete mark. When the errors where looked up it just said replace disk. Once the disk was replaced, no more problems.
Last week another customer was having the same issue as renluop. Theirs was a Dell. Luckily they had several identical PC's so I moved the hard disk between the machine and the fault followed the hard disk. Disk diags ran clean so I took a look in the event logs and found errors indicating updates issues and dump files where being created. When I analysed one of the dump files it related back to the other errors found in the logs. More importantly the dump file was telling me that the system was experiencing Video_TDR_errors. Unfortunately these errors can be caused by video hardware or drivers.
Rebuilding the system (quickest option) so far appears to have corrected the problem (fingers crossed).
renluop; you aren't my customer are you