I have had great success using a virtual keyboard called "Click-n-Type". It has word prediction and is FREE. I have students that use it for accessing word processing with head switches, round jelly bean switches and mouse clicks. It is excellent for those with fine motor issues.
With the advent of Vista from Microsoft last year, we ran into complications using the virtual keyboard. New Vista PC's couldn't run the software and when we contacted the company, they initially said it would cost too much to develop a new one.
Well, they did it! You can download it: Vista compatible Click -n-Type on their website.
This makes life easier - Thank you Lake Folks!