Did you initially register your trial software with McAfee? It doesn't appear to me that you did but this should work. Go to Add/Remove programs and remove the software and reboot the computer. Then go to Useful links at the top of this page and run the MCPR removal tool. MCPR will remove any remaing traces of McAfee that fail to remove completely from the normal uninstall and reboot again. I would then recommend you run the McAfee preinstall tool here http://download.mcafee.com/products/licensed/cust_support_patches/McPreInstall.e xe and reboot.Then insert the McAfee cd and begin to install the software and follow the directions when prompted. At some point it will ask for the CD-Key and it should at some point take to the McAfee web site to actually register your software where they will ask for your name, address, email, etc. I have always purchased my software online and thus never had a cd so the procedure is slightly different.
Let us know how this works for you.