Hello! It depends on what operating system you are using.
Have you still got the video file on your computer?
It's possible the video file that you downloaded installed its' own codec so we will need to find it.
To start with, assuming your using xp, you can right-click on 'My Computer', click the Hardware tab, select 'Device Manager' then scroll down to 'Sound, Video and Game Controllers' and locate your codecs. If you can, post me a list of what you can see.
Alternatively try this link and scroll down to the heading 'Removing a Specific Codec or MCI Device'. How to Install and Remove Codecs and MCI Devices in Windows
If you want to uninstall your media player it might be worth creating a system restore point to be on the safe side.
Start Menu>Programs>Accessories>System Tools>System Restore, follow the instructions to create a new restore point and give it an appropriate name.
To uninstall media player:Start Menu>Control Panel>Add or Remove Programs, find Windows Media Player and remove it.
You can download Windows Media Player from here: http://www.microsoft.com/windows/windowsmedia/download
Before downloading you may be asked to validate your copy of Windows.
The easier option that you might want to try first would be what I suggested in my previous post, delete the video file and restore your computer to a point before you downloaded the file.