I started using Knoppix recently, but video playback is slow and I
think it is because I do not have proper drivers for my video card in
xfree. The card is Asus Extreme X300SE/TD 128M DDR tv PCI-E. Where do
I get the Knoppix driver and how do I install it?

Are you using Knoppix from the bootdisk- or have you done an install
onto your hard drive? If you're using the bootdisk, your video
playback would obviously be VERY slow. Also, what release of Knoppix
are you using, because I believe the drivers included may have been
updated since the original release. The ASUS X300SE series card is
actually made by ATI, so you might want to try search ATI's site for
the Linux drivers. I've actually already done a small search for you,
but your card is apparently very dated (I found it for $68) and might
not be supported anymore.

You can download drivers for ATI cards here:
https://support.ati.com/ics/support/default.asp?deptID=894&task=knowledge&folderID=27