Normally uninstalling it from that machine would deactivate the item on the account but I guess you can't do that? If there's no deactivate button then contact Customer Service and they will do it for you.
It's a free call: http://service.mcafee.com/FinishInfo.aspx?lc=1033&sg=CS&pt=1&st=PHONE
Peter’s suggestion is worth a try, uninstall and reinstall the McAfee in your new PC will automatically deactivates the McAfee in the other PC. If you do not see the deactivate option kindly post the screen shot of the active machine reflecting under your account for further investigation.