First off, I'm pretty sure that the lag where people are running into walls happens when people abuse multiplayer cheats. Secondly, there's a program in the Republic Commando files called "CDKey.exe." If you installed the game to its default directory, then it'll be at:
C:\Program Files\LucasArts\Star Wars Republic Commando\Install
Find the icon that looks like the LucasArts logo and double-click on that. A dialog box will open, prompting you to retype your CD key. Type it in and hit OK. That should solve your problem.