Hello Barb, go to your hub computer and remove the computer where you are uninstalling One Care from your One Care Circle. To install One Care on the new computer download the One Care trial -
http://onecare.live.com/standard/en-us/3/default.htm To activate, open One Care, select "Subscribe" and when asked for a Live ID enter the Live ID and password associated with your One Care subscription and One Care will activate.
There is not an option to have more than three computers in a One Care Circle. Having One Care on a fourth computer would require the purchase of an additional subscription using a different Live ID but you would then have two One Care Circles.
Jim