My Yahoo password was recently cracked.
Would like to know how this was done.
Could there be a keystroke logger on my computer, or
does Avast! prevent that with 100% confidence?
yes it does stop…it detects keyloggers too…
you could use zemana antikeylogger alongside avast to be 200% sure…
What firewall are you using? Online Armor contains keylogger protection.
Private Firewall, Outpost Firewall and Comodo Firewall provide anti-keylogger security.
To test passwword strength try http://howsecureismypassword.net/ 
I used Lastpass for all my internet used with Firefox 
Check Lastpass https://lastpass.com/