Not as good as some of the antivirus programs you can buy but as good as you will find offered free.
The home version of this product is probably your best choice when it comes to choosing between the free antivirus software that is available. While it isn't the best of the antivirus scanners (you need to pay for those versions) it does do a reasonably good job of scanning your emails as they are received (the most common source of viruses). It does unfortunately occasionally report a n email as containing a virus when in fact the email doesn't (a false positive) but in many ways that is preferable to having software that does the reverse (not identifying real viruses).
You do have to register the home version of Avast even though you don't have to pay for it. This allows the authors to better keep track of who is actually using their product and certainly isn't a reason to avoid using the software. It is certainly a lot less pushy than some of the other free antivirus programs that constantly advertise the paid version of their product.
The Pro version of Avast is available as shareware giving you 60 days to see whether the addition of the antispyware and antirootkit components as well as the ability to schedule scans of your hard drive are going to be worth the cost of upgrading.