did you check your windows config..bec.if its on the “fail safe mode” options the windows drivers will load up but not all drivers are loaded bec.its more purpose of troubleshooting?next question is when the system restarts do you get any error messages?another possible reason is theres a hardware conflict between the operating system,better check w/ that areas.
Is this a new development or has it always behaved this way? Have you made any recent changes which you can roll back using system restore? On the hardware level, have you re-seated the memory yet? You might also want to try using one memory stick at a time to see if one of them is bad. If the system is under warranty get on the phone with support for troubleshooting.