ok, do the following please to make sure we have removed any corruption on the computer.
Download CC Cleaner (free) from hereCC Cleaner
(Click Download Latest Version on the top right)
Run it and let it scan and repair your registry.
Once done, restart the computer
Click Start, All Programs, Accessories, right click on Command Prompt and select Run As Administrator.
A black window will open.
Type in it
press enter and when it has completed, type in
It will complain the Volume is in use and to run on restart. Say YES and restart the computer
Than open one of the videos. Close it and tell me if its preview now appears please.