I've been using the free version of Avast for years with no problems. They have a free version you can use permanently, you just have to re-register each year. If you were using a free trial of a paid version, just go to the actual free version. I don't have any personal experience with Kaspersky, sorry.
As Entomber said, paid versions are preferable to free but what's more important is user education. Users are usually hit by viruses because of malicious emails, visiting malicious sites, downloading infected files, or using infected media. You don't need expensive anti-virus software to avoid these things. If you can't identify where an email came from, don't download attachments or click links in it. Malicious sites are harder to...