Rundll32.exe is a process that can call dynamic link library functions from the command line.  It is used extensively in the windows control panel and is called during windows startup by a number of processes.
You need to find out when it goes to 50% (during boot, after running some software etc.) and find out what process is calling it.  I have had older software cause this and had to uninstall it.
"is it normal to have rundll32.exe-257618f.pf and many other rundll32.exe-xxxxxx in the c:\windows\prefetch dir? "
I have them in prefetch too so I guess its normal.