You can't stay safe online without powerful security software on your computer. Fortunately, powerful doesn't always equal expensive.
avast! Free antivirus blocks viruses and spyware. The free version also includes a remote-access feature that allows a friend to help you out.
The paid version has a more powerful sandboxing feature, secure online payments, a firewall and spam-blocking tools. The free program should be enough for most users, though.
During installation, avast! will try to install Google Chrome, which is one of my favorite browsers. If you don't want to download it, however, make sure you deselect that option. It also requires e-mail registration.
The free version is intended for home use only. Don't use it for your business computers.