Rundll32.exe is a process which executes DLL's and places their libraries into the memory, so that they can be used more efficiently by applications. This program is important for the stable and secure running of your computer and should not be terminated. Check and ensure that the particular rundll.exe is situated in the System32 folder.

The following is, primarily, based on the fruitful discussion at TechNet, and the important points have been culled out from there and presented here.

If you get the error when opening folders that contain video files, try this and see if it helps:

Open Control Panel > Folder Options > Always show Icons, Never Thumbnails

Else, try to remember, when did this error start popping up.
Was it after installing a particular program? Was it after installing, say, Nero ? if yes try this:

Open Computer > System Drive > Program files > Common Files > Ahead > DSfilter.
Rename "NeroVideoProc.ax" to say "1NeroVideoProc.ax"
Also rename "NeVideo.ax" to say "1NeVideo.ax"
Reboot.

You can also follow these steps to try and find out, which software is causing this issue:

Type msconfig in Vista's search box and hit enter to open up the System Configuration Utility window.
Click on Services, and check in the "Hide all Microsoft services" checkbox.
Click on "Disable All" button.
Now click on "Startup" tab, and then click on "Disable All" button again.
Press OK button and then "Restart" button on the following window.
Effectively, you have turned off all non-microsoft services and startups.

If the issue is resolved, open System Configuration Windows again and then enable all the services from Services tab and then check the issue after restarting the computer in the same way as mentioned above.

This way, you will have to enable/disable services and startups to narrow down on the culprit. Agreed, this is a lengthy and painfully boring process, but it will help you identify the culprit.