Good solutions IMO. All three are great in their own ways, here's my take on each of them.
Avast free
PROS
Easy to use and configure
Is a lightweight AV
Avast has pretty fast scans
Great detection, and very secure on hardened mode
Works well on old PC's and only requires P4 CPU with at least EE2 instructions.
CONS
Heaviest on resting RAM usage but still only peaks at 43ks just like Windows Defender
Installer I recommened the custom install! Lose all that bloatware, Avast only
Privacy policy is generous to Avast
Needs registration
Bitdefender free
PROS
LESS fuss then both Avira and Avast
Requires NO fuss at all (install + forget)
Very light on system
Doesnt scan the same safe file twice
Has a great virus shield on it (detections are great)
Runs well on OLD hardware just like Avast
NO pop up, PUP's or adds at all!
CONS
Bitdefender is a Buggy AV free and paid versions
Weird bare minimal UI, example = (right click "B"itdefender logo to find the full scan option)
Average zero day
Updates can be a bit heavy but are somewhat quick
SLOW as HELL scan speeds
User requires email to stay "logged in" and running
Avira free
PROS
Probably the best privacy policy (respects your info)
Easy to use and light UI
Light on system, as are the other three
Great signatures
Fast scanning
No user sign up or email required
CONS
Poor zero day
Cluttered and old looking Interface with many confusing settings
Needs a good firewall to go with it