If your OS was a Linux derivative, I doubt you'd have this problem. So, with Windows . . .

1. Do you close your browsers before going into "sleep" mode? Not a good idea to leave them open, rouge scripts could be running in the background. Could cause what you experienced.
2. How about other programs or games?
Any program (or even programs running in the background) left running could have/create a slow memory leak (slowly use up more and more memory). This could be your gremlin.
3. Windows updates. Based on your time between shutdowns, Windows could have pushed a patch or update that might cause a reboot to be necessary for normal operating.

Truly, OSX (sort of a Linux second cousin now) is better and most Linux distributions even better at lengthy, stable periods without reboots.