I believe that Avast is every bit as good as Norton's if not better.
I find that Avast is less bloated (in code) and uses less resources to run than Norton. In otherwords, Avast is efficient AND effective.
Avast auto-updates itself regularly, such that virus etc detection is excellent. Also, the great people on this forum, including Avast employees, are very helpful and provide quick feedback. You can't beat that!
As an aside, I know people who've run Noton AV only to see a virus etc basically trash Norton's (rendering it brain dead) while I've never seen that happen to Avast.
So I wouldn't hesitate to install Avast if I were you.