Actually from what I've read about battles in this period, knights fought dismounted just about as often as they fought mounted - especially early in the period.
One of the reasons being, that horses where expensive.

Of course this didn't happen late in the period, when knights were dressed in full plate - that would have been impossible to walk around in....

In many battles about 1/3 of the total mounted forces where held back as a reserve to be inserted either where the there was danger of breaking, or for chasing the routed enemies.

[This message has been edited by dagdriver (edited 01-01-2002).]