Marcie, I'll try to post in a simple way:
1. Desinstall your actual version of avast (Control Panel).
2. Boot
3. Use
Avast Uninstall for complete desinstallation.
4. Boot
5. Use the Control Panel > Install/Uninstall Applications applet to remove Norton Antivirus (NAV) (it´s not necessary to remove other applications from the Norton SystemWorks not even LiveUpdate or LiveRegistration).
6. Boot.
7. Use
RNAV2003 or
this link to download and then remove traces of NAV into your registry. Download and run the application. If you have already done the first step, choose 'No' to continue.
8. Boot.
9. Do not remove manually or using another software the Registry keys, it´s not necessary (by now) and may be dangerous. But, if you want and do not fell unconfortable walking through windows registry and deleating the many left behind entries, there is some good registry cleaners available to aid in complete removal of these.
Reg Cleaner is a good one but it is a Shareware (you can run once to do its job). Another links and applications could be found
here at the section
Registry Tools. For Windows 98, I recommend
RegClean.
If you have time, you can read more at this
Windows Registry Guide.
10. Install avast! (answer 'Yes' for the presence of another av, in this case traces of NAV). Starting with version 4.0.172, there is a functionality in avast! Setup to detect and warn the presence of Norton Antivirus installed on the target machine.
11. Boot.
12. You will have to change Registry "corrupted" keys by NAV, see
here.
13. At last (after the boot), make a eicar.com test (see links
here).
Hope this help.