Well, that would depend on what OS you're using and what hardware you have...
Generally, you can search for the codecs by the file extension you're using. Fof the drivers, you need to go to the video card manufacturer's website and look up your exact card and OS and download the latest one.
Go into the device manager (right click on My Computer => Properties => Hardware => Device Manager). Look up what you're video device is. Without that, you're not going to be able to do much.
For the codec, look at the file extension (i.e. .avi or .wmv or whatever) at the end of the file name. You may have to change your view to see it. Then Google that extension with the word "codec". Download one from a known vendor (i.e. Microsoft). Install it and see if that works.