Avast is definitely good choice. It is quite stable product, great detection/removal, nice features, good UI and some other great things.
But if you only scan computer with Avast then there is still a chance that PC is not clean.. Solution is to use some other on-demand scanners too (e.g. HitmanPro, MalwareBytes,...). There is no best product...
Sorry about off-topic:
1. ESET, amazing product (performance, detection, tests, reviews, features)
2. Avast, good freeware choice (from the paid products I would buy ESET)
3. Kaspersky, great detection but performance is not that good
4. AVG, was good product, don't like current version
5. Microsoft Security Essentials, maybe it is the best choice for the average/novice users (simple, easy to use, detection is not that good but it should detect important infections)
Regards,
Kardo