windows 7 - Win 7 - computer locks up when dragging large video files in explorer or closing them in WMP?

I have a library of 720 and 1080p videos of my son on my computer (Q6600, 3gb ram, ATI 2600 xt, up-to-date win 7).

The problem lately, is that if I view a video, it plays fine. However, once I close WMP, the computer locks up completely (no ctrl+alt+delete, nothing at all). I have to force shutdown. I get the same behavior if I drag files in explorer. The relation between this and WMP is that it locks up if I have large icon previews turned on (essentially loading up the video in the explorer icon preview). So, I'm guessing these two are related.

I've already ran a disk error check, and it's defragged often. Everything is up to date, even GPU drivers.

Also, I do use the K-lite codec pack, if that's any consideration.

EDIT: Also, the video files that cause the lockup are MP4 and MTS.

Any ideas? Thanks.