Is there any way to collect the PC username / log on name as part of the form dataset using Javascript?

Javascript only has access to the browser DOM and I don't beleive that any type of windows specific information is available to it. If you building something that's going to be used with I.E. on a windows platform you might be able to find something in a JScript (Microsoft's JavaScript) reference that is specific to accessing I.E. information.

I agree with planoie - client-side javascript is not much use for this

if you use NTLM authentication then the browser will send the username to the webserver. If its possible for you, you could turn your HTML page into, say, an ASP page and populate a javascript variable with the username, from Request.ServerVariables("LOGON_USER"). Then you would have the logon name available in your client-side js.