I know much better solution (without any F8 key)
METHOD I
1.Run cmd.exe with administrative previlliges (right clik as admin on this file) and type
bcdedit /set testsigning on
+ restart
2.After restart you can now install driver (it is only possibly if driver is already non-credential signed - you have to split non-credential signing and MS signing) and remove watermark - because you'll have on screen title "Test mode"
TCP-Z, Best TCP/IP Patch: Remove Watermark, V0.8, Build 20090509
METHOD II (with non-credential signing also and interesting lecture)
Driver Signature Enforcement Overrider 1.3b - NGOHQ.com
I my opinion METHOD II is better
FYI
ver 1.3b is fully compatbile with UAC