It may happen when you open a particular folder specially when that folder contains videos (.avi, .flv) downloaded from net. These files lacks properties within them, and explorer tries to get properties with its inbuilt properties handler. You can solve this problem by disabling windows desire to "decode" the file when reading it. I think it is actually trying to determine width, height, etc. This reg tweak makes windows stop scanning the avi files.

1. Open up regedit
2. Goto HKEY_CLASSES_ROOT\SystemFileAssociations\.avi\shellex\PropertyHandler
3. Delete the "Default" value
4. Restart the system.

hope this helps.
This tip (which will probably fix it) will remove some functionality from windows explorer though, more specifically if you try to use the properties menu of the file to identify the codec, or check it's resolution then you won't be able to do it with this key removed from the registry.

Make sure you export the key before deleting it so that you can restore it if you want to.