You don't need to deactivate avast to download or run the avg uninstall tool.
You should first have used the add remove programs to remove avg, before installing avast. If you didn't do it that way you may need to reinstall avast after you have run the avg uninstall tool and rebooted. But check first to see if avast is running as it should you may not need to reinstall.
Where you experiencing any problems with avast that sent you in search of the avg uninstall tool (or what made you go looking) ?
I presume this is the tool you are talking about - AVG Remover, download tool from here,
http://www.grisoft.com/ww.download-tools there is a 32bit and 64 bit windows version, ensure you use the correct one.