Noted that "katep don't" above found system restore did not resolve the issue. To add more to the mix, I am working on a laptop for a friend - no Quicktime, just iTunes, same problem. MOVs from iphones causing problems.
I have found that block deletes of multiple files can be done. CTRL A, delete key, then empty recycle bin from icon context menu - any attempt to access recycle bin will fail.
Have also noted that the only MOV Explorer has created a thumbnail for is the one I accessed when creating the association with itunes.
I have managed to associate MOVs with iTunes, which seems to help in playing some of them, but explorer crash problem remains. iTunes version: 12.0.1.26; Win 8,1 fully up to date on Lenovo c500. iPhone is the 3GS.
Might be informative to share specs here, see if there is any closer common factor.