Hi Gary,
If you load that existing Web Form and go to step 8 of 9 look for the Activity - AssignedTo in the Table - Field column and edit it, you can change the user that the activity is created for. Since this is a hidden field and is not displayed on the web form itself it should not require any changes to the web form code on your site.
-Michael
Michael Melo
Product Manager
Method Integration Inc.
Website: http://www.method.me
LinkedIn: http://www.linkedin.com/in/MichaelMelo