I'm betting you are typing on a laptop. Am I right? If so, this is a big
problem that lots of people have, and its caused by the heel of your thumb
brushing the touchpad and generating a 'click'. It makes the cursor jump
back to wherever the pointer is at the time, which is normally the beginning
of the text field you are typing in, or the previous one.

It's a bloody nuisance, but not Vista's fault.

See if you can reduce its sensitivity. There is often a setting for that
somewhere.

SteveT