This question should be asked in the Windows Answers Forum at the following address
http://answers.microsoft.com/en-us/windows
Sometimes a problem lie this could be due to a virus or to a hardware problem, like overheating or overclock.
I suggest you press F8 when the PC starts and to try booting using the Last Known Good configuration; if this works fine, enter the Windows "System" log in the Event Viewer and check for some records related to this problem.
You could also ty reverting back your system to a previous well functioning state by applying a restore point.
Bye.
Luigi Bruno - Microsoft Community Contributor 2011 Award