Question

some how this file has been deleted from my computer c:\windows\rundll32exe and i dont know how to reinstall it.

Answer

The rundll32.exe is an important system file and if it's deleted it's likely other system files have been deleted. You can try manually extracting this file from the Windows CD. Additional information about how to do this can be found on the below page.

http://www.computerhope.com/issues/ch000363.htm

If this does not resolve your issue or after being done reports more missing files. We strongly suggest you simply reinstall Windows. Additional information about erasing the computer and starting over can be found on the below page.

http://www.computerhope.com/issues/ch000186.htm