When dealing with a similar problem over a period of time like you discribe, there was a message related to "explorer. exe, Application Error,Faulting module to ntdll.Dll
I ran the sfc /scannow with OS cd in, which appeared to cure it,or it may have been all the windows updates that i loaded at the time.
If you go to start, right click on my computer and click manage, and when management window opens up click on event viewer.Click on application and scroll down looking for the red X ERROR. Right click and then properties and have a look at the report.You SHOULD see the reason for the error at that time. You could do the same with all listed.
If you followed it up by clicking on the web link in the window (if on line at the same time) and selecting ok you may end up with this or similar below being sent back to you,depending on the error.
Product: Windows Operating System ID: 1000
Source: Application Error Version: 5.2
Symbolic Name: ER_USERCRASH_LOG
Message: Faulting application %1, version %2, faulting module %3, version %4, fault address 0x%5.
The indicated program failed. The message contains details on which program and module failed. A matching event with Event ID 1001 might also appear in the event log. This matching event displays information about the specific error that occurred.
No user action is required.
+ Related Knowledge Base articles
Saying that trying to fathom it out,is time consumming if at all poss. :)