Disable any AntiVirus and ensure that your firewall is allowing EMM to connect through it.....So if that doesn't work for you then go to the download page and download a full install http://www.binaryworks.it/extrememoviemanager/download.aspx reinstall it and make sure that your update works after. The full install has all the updates, but you should still be able to run updates. I am hoping that the reinstall fixes the firewall permissions for you (you are installing as administrator correct?) Also I believe version 8 is still considered Beta
If Windows can't find the old install you "could" manually delete it and run a registry cleaner...but check where it is installed (Should be C:\program files\eXtreme Movie Manager or C:\Program Files (x86)\eXtreme Movie Manager 8\ depending if it is 32 bit or 64 Bit Windows. Running the new install should put it into the same location and overwrite the files. If you installed as the user and not administrator it may have put it into a different location which is why Windows can't find it to uninstall?