The license agreement is the part that most people ignore during an installation, but I tend to read them on principle.

Then I saw this.

Anti virus softwares have always been used to scan the potential hazards on your computer. That only applies when you already have a certain virus, spyware, or malware downloaded or transfered to your PC. Now, however, you can avoid them being on your computer in the first place by pre-scanning files before downloading them. With Dr. Web’s Firefox, Internet Explorer, and Opera add ons, downloads can be scanned first before actually being downloaded. This can avoid major problems that may occur with viruses actually being and running on your hard drive. So really, just avoid downloading crap by scanning suspicious files with Dr. Web…

Dr.Web [Firefox Add-ons via the How-To Geek]
Dr.Web® Anti-virus Link Checker for MS Internet Explorer [Dr. Web]
Dr.Web® Anti-virus Link Checker for Opera [Dr.Web]