I have tried finalcodec and other codecs. It doesn't help. It is really weird that both the video and the slider bar flashes. Even I have downloaded the perfect codec, it cannot solve the flash of slider bar. I give up.

Try different ones and see if any of them improves the situation.

What codec should I use?

> I have downloaded this codec "K-Lite Codec Pack Basic 10.4.5"

It's not a codec. It's a codec *pack* (many codecs bundled). And unless you specifically tell it to use an alternative codec (during setup or by using one of its configuration tools) -- specifically DirectShow filter -- for WMVs it won't change anything.

I have downloaded this codec "K-Lite Codec Pack Basic 10.4.5".

I did not point you to any codec download. I pointed you to a tool that allows you to manipulate your system's existing codec settings. You still haven't specified exactly what you did. Since I have virtually no information to work with, I'm afraid I can't offer further suggestions at the moment.

It is from the same place that you offered. Believe me, I have already asked many people to help include the experiment assistant in the lab. He told me maybe I should choose another software to fix this problem, for example, presentation or pyschopy. That is my last option, I am still looking forward your suggestion.

Which codec? From where? Please be specific. And there should be no reason one would have to download a codec for properly encoded WMVs on Win7 -- Win7 has those natively. It is thus unlikely that whatever you did would have any positive effect, it might even make things worse.

I have already done this yesterday. Download the codec from internet and install on the computer. It doesn't work.

> I have already tried the codec yesterday. It doesn't help.                                What exactly did you do? It's entirely unclear what "trying the codec" is supposed to mean.