I've found this solution myself like 2 days ago, started playing L2 again and found out that IG Walker wouldn't start on my Windows 7 installation. So here it goes, it's really simple and you don't have to edit ANY files!
Step 1:
Download the IG Walker for your chronicle, I used L2Walker for Gracia Final, on RPG-Club.
Step 2:
Download Wininject: http://rapidshare.com/files/351224562/Winject.rar and extract it anywhere (or just in your L2Walker folder to make it easy)
Step 3:
Open Wininject, now press on "File" -> "Select File" and search for L2.exe of your client. Now under "DLL to inject" in the main menu, go to the folder where you installed L2Walker and select L2Walker.dll (or LineageII.dll, depends on the chronicle).
Step 4: Press launch ;)
NOTE: You still need to use the hosts file and/or run a seperated file to activate the walker if it's not pre-cracked!
Enjoy ;)