You must unninstall Norton, and use it's unnistall tool after that to clean the system from it's remnants, avast! will not work with another antivirus present in the system.
Sorry for the late reply, but agree with the above. You cannot run 2 AV's (or disable part of an AV and run another AV...basically having 2 AV's in your system) at the same time because this will cause conflicts. Therefore you need to uninstall Symantec via their uninstaller tool:
http://us.norton.com/support/kb/web_view.jsp?wv_type=public_web&docurl=20080710133834EN, then reboot your machine.
Disabling is not the same as uninstalling.
Since Symantec is known to leave remnants even after using the uninstaller, I also suggest that after running their tool and rebooting that you go into Safe Mode > Advanced Search >
enter "sym" into the search box > and again type in symlcbrd.sys (which is a Symantec Core Service BIOS reader) and if anything comes up you can delete it. Repeat the process third time, but type in NIS or NAV (what ever you had on your machine), and then type Symantec and see what entries come up. If in doubt of a file, right click on it for Properties to see if it is a Symantec file...if so you can delete it and empty your recycle bin while in Safe Mode, then restart your machine.
If you have a cleaner on your machine, like CCleaner Slim
http://www.piriform.com/ccleaner/builds, run the cleaner and registry cleaner (you can make a back up of any registry fixes in My Documents temporarily) after doing the Symantec uninstall. Most of us here use CCleaner. If you did end up removing a lot of Symantec files, you may also want to consider defragging your machine to free up some space as well, then reboot.
Should Avast give you any problems after this, since you did have 2 AV's on your machine, try an Avast Repair first:
- Go to Control Panel > Add/Remove programs > Avast Antivirus.
- Scroll down and choose Repair function in the pop-up window.
- Reboot.
Let us know if you have any questions. Thank you.