> I am wondering why certain video files run fine on windows media player or vlc but not when in Inquisit.
Let me try to explain:
VLC comes bundled with its own set of codecs (which are used only by VLC and are not available to any other applications on the system). So, a video that works fine in VLC may very well not play in any some other application that relies on other or system-level codecs.
Inquisit 5 relies on system-level codecs, specifically Inquisit 5 relies on the Microsoft Media Foundation (MMF) framework (available and gradually extended as of Windows Vista) to render videos. The advantage of MMF is that it supports a number of common and modern video formats out of the box on current Windows versions, although it is lacking support for more exotic formats.
Inquisit 4 still relied on an older, different framework (DirectShow codecs). A vanilla Windows installation comes with relatively few DirectShow filters, and to use MP4s, for example, you'd often have to find and install a suitable 3rd party codec first.
Now, Windows Media Player can (still) use both: MMF as well as DirectShow, and which one it uses will depend on the video format, which type of codec for it is available, and settings buried pretty deeply within Windows. This explains why a certain video might play fine in Windows Media Player (per a suitable DirectShow codec), but may not work properly under Inquisit 5 (because the video isn't fully compatible with MMF).
I know this is all a bit technical, but I hope it's still somewhat useful.