Rundll.exe loads and runs 16-bit DLLs, and rundll32.exe loads and runs 32-bit DLLs.
There is nothing wrong with having them on your system so long as they are both the actual microsoft product.
If you have an XP system and you see plain rundll.exe, it usually means you have an old 16-bit program on your system that you run OR sometimes spyware. BIG sometimes, it's very unusual.
If it appears after re-boot without running anything I'd start checking my system. If it appears after running an old program it's pretty much normal.