Multiple Sclerosis (MS) is a disorder of the body's immune system that affects the Central Nervous System (CNS). Normally, nerve fibers carry electrical impulses through the spinal cord, providing communication between the brain and the arms and legs. In people with MS, the fatty sheath that surrounds and insulates the nerve fibers (called "myelin") deteriorates, causing nerve impulses to be slowed or stopped. As a result patients with MS may experience periods of muscle weakness and other symptoms such as numbness, loss of vision, loss of coordination, paralysis, spasticity, mental and physical fatigue and a decrease in the ability to think and/or remember. These periods of illness may come (exacerbations) and go (remissions). Fampridine-SR is an experimental drug that increases the ability of the nerve to conduct electrical impulses. This study will evaluate the effects of Fampridine-SR on the walking ability of subjects with MS, as well as to examine the effects on muscle strength and spasticity. The study will also examine the possible risks of taking Fampridine-SR.