Not directly from my mouth, copied from a website on google
Found the supposed uninstall info in HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Uninstall\WgaNotify
The WgaNotify installs with NoRemove DWORD value in the registry at 1, preventing it from appearing in the Add/Remove programs list, but it doesn't actually have an uninstaller string, which would be in /WINDOWS/$NtUninstallKB905474$/ but seeing as that folder was never created, it cannot be uninstalled the conventional way.
I found the update.inf for it though, listing all the reg modifications and files copied across.
It installs these files:
system32/LegitCheckControl.dll (an update for the original WGA Tool)
and acts as a direct hook for the winlogon.exe file itself.
I found this info in C:\WINDOWS\SoftwareDistribution\Download\6c4788c9549d437e76e1773a7639582a\update\
So to remove the whole thing, it probably would be an idea to delete the registries + all 5 files, making sure nothing is left behind, and then to re-install the old WGA Tool without the new notify update.
You can remove the hook from starting up using Autoruns by Sysinternals, browsing to the Winlogon tab.
If any of the files are locked to prevent deletion, just use a free program called Unlocker to remove it from the running processes.
Also, make sure you remove the WgaLogon key in HKLM,SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\Notify\
After that'* done, do a reg sweep and delete all instances of WgaNotify in your reg.
C:\windows\system32>taskkill /F /IM wgatray.dll && del wgatray.dll
C:\windows\system32>del wga*.* LegitCheckControl.dll dllcache\wga*.*