Do you have Photoshop installed?
Right click on a jpg and choose properties.
Then go to the Photoshop Image tab
and untick generate thumbnails.
If you don't have that Photoshop Image tab. Then it must be some other program messing with your thumbnails. Try to think of when the problem first started and if you happened to install any image programs around that time. If so, uninstall them and see if that helps.
With the problem where explorer crashes on avi files.
Only do this if you are happy with Explorer not being able to show the first frame of video in thumbnail view. (This is what causes it to crash if it sees a corrupt one.)
Go into regedit
That's: Start>Run>type regedit >hit ok>
In regedit expand HKEY_CLASSES_ROOT
Scroll down to SystemFileAssociations, expand that
Then find .avi, go into shellex then PropertyHandler.
In PropertyHandler you'll see something like this:

Select it and and delete it.