Do this. It solved my problem quick and easy. This replaces your bad rundll32.exe with a new one.
Put the Windows XP CD ROM disk in the CD ROM drive.
Click Start, and then click Run.
Type expand X:\i386\rundll32.ex_ c:\windows\system32\rundll32.exe in the Open box, change X to the letter of your CD ROM Drive. Run it.
Restart the computer