I've been in the IT industry for over 14 years and done my share of resolving issues stemming from online threats, including virus removal and dealing with other types of malware. Avast is a fine product and I've recommended it to many people over the years.
For a long time, I've only used free antivirus products along with the built-in Windows Firewall... I use Microsoft Security Essentials (MSE) on my personal computers and our company uses Microsoft Forefront Security (client and server), which is basically the corporate version of MSE. I've used Avast! in the past and it's a good product as well - just seems that MSE is a bit more lightweight and less obtrusive to my computing experience.
In the meantime, when you say that "viruses are ruining my PC", you may want to try Malwarebytes Antimalware. Select the free option at the end of the installation wizard by opting out of the "free trial". Update it's definitions database and perform a FULL system scan. Remove anything it detects, reboot and you should be in a better situation than before with regard to these viruses ruining your PC.
Of course, no software can replace common sense. While there are many anti-virus, anti-malware, anti-adware, anti-spyware, etc software products out there, the best tool is knowing what to avoid and how to handle things like email attachments, pop-up windows, ads, and information from unknown sources. By coupling this knowledge with free anti-virus software, I've not had a single virus on any computer I've personally used in over 10 years.
In fact, I created an ebook which teaches people how to do the same thing I've done using only free software while also educating them by proving easy-to-understand tips. I'd love to get feedback on it from anyone reading this forum. You can find it at http://www.howdoyougetmalware.com