I encounter a problem installing WinPcap in windows 8 when I try to install GNS3. So I search and found the resolution for that. Make sure You download WinPcap. Go the folder and right click the WinPcap_x.x.exe file then go the properties, on properties window click on Capabilities tab, now check on "run this program in compatibility mode for:" choose windows 7 from drop down menu. See the picture below. click on red circle to install the WinPcap. While installing WinPcap if error message occurred " Error: "An error occurred while installing the Microsoft Network Monitor Driver (NetMon) (0x800F0203). You will be able to use WinPcap on standard network adapters, but not on Dialup connections and VPNs. Please contact the WinPcap team." download Microsoft Network Monitor tools, from Microsoft site and install. Now you can easily install the and run the tool.