Robert,

           Once you have the machine "whipped into shape" Microsoft Security Essentials is just fine as an antivirus.  I would also suggest installing Spywareblaster and Spybot Search & Destroy, being sure to use the immunization feature of the latter.  Both Spywareblaster and Spybot use pre-emptive measures to prevent clicking through to known problem sites.  I haven't used Spybot in a while, but keep Spywareblaster on all of my machines.

            I cannot speak to accessibility of either Spywareblaster or Spybot, as I've not tried them with a screen reader.  With any luck someone else here has.
--
Brian

 Life is the art of drawing sufficient conclusions from insufficient premises.

         ~ Samuel Butler, 1835-1902