Method 1: If You Are Able to Run Windows
1. With Windows running, insert the Windows CD or OEM recovery CD into your optical drive.
2. Click the 'Start' button, and then select 'Run.'

3. Type the following command in the 'Run' box (X is the letter of your CD/DVD drive; check your system to find the proper drive number):expand X:\i386\rundll32.ex_ c:\windows\system32\rundll32.exe
4. Reboot the computer. Your corrupted rundll32.exe file has been replaced with the original.
Method 2: If You Are Unable to Run Windows
5. Insert the Windows CD in your computer's optical drive. Reboot the computer.
6. Upon restart, you may be prompted to 'Press any key to boot from CD/DVD.' Follow these instructions to boot from the Windows CD.
7. You will see a 'Welcome to Setup' screen, which presents you with a few options: 'To setup Windows XP now, press ENTER,' and 'To repair a Windows XP installation using Recovery Console, press R.' Even though you are performing a repair installation, do NOT press 'R.' Press 'Enter.'
8. Select the Windows installation you want to repair from the list that appears. Look for C:\WINDOWS 'Microsoft Windows ...'. Then press 'R' to enter repair mode. Press 'F8' to accept the Windows licensing agreement.
9. Windows will reinstall the operating system, overwriting your corrupt or missing rundll32.exe file. Your other data and files will not be erased.