If by occasion you erased valid file which you thought was from missing rundll32, you can repair it from your system CD using sfc.exe utility. Competition with rundll32 exe virus in speed may give you a sad result. If you found Rundll.exe launched in autoload advances list it's the most possible sign of infection and you should fix rundll32 exe.
Do not mix missing rundll32 with the legal system file called Rundll32.exe which is always located in your Windows\System32 folder. Look at your previous infected directory and get sure rundll32 exe virus is already there. After you get assured that Rundl trojan has happened on your system it is recommended fix rundll32 exe.
