Rundll32 is a generic process that is used to launch other applications (usually DLL's). Best way to work out what it is that's causing this is to have a look through the started applications for that user (hklu\software\microsoft\windows\current version\run) and see if he/she has anything in there that's using rundll32 to execute things at startup.
Other than that you could log into that user, bring up task manager and kill any rundll32 processes and see what applications have stoped running (check the tray icons out before and after you kill the process as it's likely to be one of these).
Also it's probably a good idea to run a spyware/ad scanner on the machine see if it's anything to do with that, my personal favorite is SpyBot 1.3 (freeware) just google for it.
Hope that helps
