1. I suggest you desinstall avast from Control Panel first.
2. Boot
3. Use
Avast Uninstall for complete desinstallation.
4. Boot
5. Install McAfee without any fear, it will work...
If you really want to be sure of a complete registry cleaning, could be useful one of these Registry cleaning applications:
RegClean for Windows 98 (but works on XP)
BeClean Specially the Registry Cleaning feature
But the best, you can make a download for try, is
PowerTools. Very powerful registry power tools, with lots of extra options. Unfortunately, it's a shareware. You can run it before the expiration time and get rid from the previous antivirus application. You can choose an option for backup of the removed itens.
My suggestion, anyway, is that you ask your McAfee money back!