Maybe a couple if things. If the device is slowing down the system, possibly it is having problems communicating with the router. What router do you have and is it configured for 802.11n? Both devices may be backward compatible, so it will probably work, but maybe not as well as it should. Also, if it is working on the wrong internal network, or using the Microsoft supplied IP address, it might be slow, so make sure you check the IP addresses it is using.
Do you have Mac Filtering on your router, and if so is the new MAC address allowed, and set for the proper security. It would seem if the adapter was seeing the network at all, the Mac Filtering has to be set up correctly--assuming it is looking at YOUR network. Click the Wireless icon and see if your network shows up and what signal strength.
Not really sure as to what effect the anti-virus might have during an install, but it seems strange you would have to go into safe mode to install it. You can select the installer to run as admin if that would help. I also do not understand your comment about double clicking to enable. I right click and select enable in the Network and Sharing Center Adapters window. But is should have installed as enabled, so maybe the fact it may not is part of the problem.
If you cannot get it working to your safisfaction, I would suggest you go into Device Manager and uninstall the drivers. When it asks if you want to uninstall all instances, select yes. Then reboot and try installing the drivers again with the anti-virus off if necessary. If you can, select the Run As Administrator option.