http://ask.wireshark.org/questions/1281/npf-driver-problem-in-windows-7
應該是我換了新版的Wireshark和Wincap後，npf被設定成了demand start，
參考上面連結，很清楚的可以設定為auto-start。
Run a cmd.exe as administrator and run the command sc qc npf.
You should get some output like this:
C:\Windows\system32>sc qc npf
[SC] QueryServiceConfig SUCCESS
SERVICE_NAME: npf
TYPE : 1 KERNEL_DRIVER
START_TYPE : 2 AUTO_START
ERROR_CONTROL : 1 NORMAL
BINARY_PATH_NAME : system32\drivers\npf.sys
LOAD_ORDER_GROUP :
TAG : 0
DISPLAY_NAME : NetGroup Packet Filter Driver
DEPENDENCIES :
SERVICE_START_NAME :
If your driver is not properly started, activate it with the command sc start npf
Finally, to start the service automatically, use the command sc config npf start=auto
Remember to run your cmd.exe as administrator when issuing these command.
