rundll32.exe definition, relationships, removal:

Rundll32.exe loads and runs 32-bit DLLs. Rundll32.exe comes with all versions of Microsoft Windows. If you are worried if this process is legitimate, check the .dll name that follows rundll32.exe.

Spyware, adware, keyloggers, trojans, etc often hide behind the rundll32.exe process. rundll32.exe is located in "C:\WINDOWS\SYSTEM\" on Windows 95/98/ME, "C:\WINNT\SYSTEM32\" on Windows NT/2000 and "C:\WINDOWS\SYSTEM32\" on Windows XP.

Our advice is:
Open command prompt and write:

tasklist /m /fi "IMAGENAME eq rundll32.exe" >C:\rundll32.txt

This will write into rundll32.txt file any unusual .DLL files used by rundll32.exe.
It is best to see the relationships and deside if suspected file is really suspicious. File rundll32.exe is related to trojan StartPage. File rundll32.exe is related to trojan ZenDown. File rundll32.exe is related to trojan Rebohon Trojan. File rundll32.exe is related to Wowcraft.