Math Geek :
working with old win xp systems with less than 1 gb of ram (512 was the norm) showed bitdefender using too much. right now avast is using 21 mb on my system. bitdefender was well over 100 mb and stopped these old pc's completely when scanning an item. avast slowed it down a bit but did not stop working to wait for the file to be scanned.
I don't know why this is with older hardware... I'm guessing it's due to drivers, or some compatibility problems. My dad's computer had a problem running BD, and I could never figure out why. It still has some serious issues, though... so I can't say that BD is responsible for the few problems it "caused"...
Math Geek :
as for false alarms and vulnerabilities, it's little worse than the other top rated programs. as i skimmed review after review for ideas to try out, avast was just a step behind the top rated ones. such as "it caught 95% of the viruses but _____ caught 97%" so yah it was not as good but the difference is not that much if resource usage is a concern. 21 mb for 95% vs 250 mb for 97% is a big difference overall.
That really isn't what I'm talking about. Check out AV-Comparatives's False Alarm test; that covers the false alarms I'm referring to (different from false positive results for viruses). While Avast may catch a competitive amount of infections, catching infections is only part of the battle. Also, their live protection is lacking. I haven't seen too many in-depth reviews on this, as most reviews cover manual scans (which yields entirely different results). Countless times I've seen Avast not catch an infection with it's "live protection," but catch it after the infection has taken root. Definitely not a software I condone, for many good reasons.
But, if it's the best one you have available, go for it. I'd definitely recommend it over MSE.