Vista - unable to install antivirus

I had a Norton antivirus that came along with the laptop when i bought it but it has expired so i removed/uninstall it and tried to replace it with AVG but I wasnt able to do so due to the error that says i already have an antivirus and i should remove it first before installing a new one. But i already removed the norton and there's no other AV installed in it. I only have windows defender but its just a spyware right?..

another question, is it ok to have another spyware aside from windows defender?

thanks a lot for your help!

Try uninstalling again using the norton disk in the administrator account.
You can have more than 1 anti spyware running but I would not bother. Running a spyware checker once a week on all files should suffice, if you have a spyware checker running all the time in the back ground. Use a different spyware checker to scan all files from the one you use all the time. This is because different spyware checkers have different levels of protection. I would recommend spybot search and distroy.

Norton, even after removal, leaves program fragments installed and running. Use the following to uninstall Symantec's Norton.