I've put 'some' time into learning to test accessibility and so far Bobbie
seems to be 'the' software tool most appear to be using. I've put some time
into trying to actually use pages I've made accessible and while validation
is important for certain types of sites what is more important are the
usability factors. Thus, I'm planning at some time to buy Jaws which is the
most widely used screen reader. Usability is IMO more important than
validation.

Start for example with the Windows Help and enter 'accessibility' and learn
about the Accessibility Wizard, notably the Narrator. Use the narrator (with
your eyes closed) on some of your accessible pages and you'll immediately
know what I'm trying to convey. Usability is critical.

You should also go to W3C website [1] and check out their accessibility
initiative for links to resources.