There is no practical way to absolutely guarantee saftey from infection. What you can do though is to minimize risks and lower your chance of infection. Viruses have shipped with vendor supplied media, and there are even cases of infected picture frames shipping.
Infections have many sources, ranging from legitimate sites that have been unknowingly hacked, to infected ads, to sites that shouldn't have been visited in the first place such as pirated software sites and porn sites (if you wonder if you should go to a particular site, you probably shouldn't). Many questionable sites can infect you simply from visiting the site; you don't need to download anything to become infected.
There are many ways to increase your protection.
- Install a HOSTS file like MVPS HOSTS file, and keep it updated.
- Run an antispyware protection program like Windows Defender (it's free).
- Always run an antivirus program and a firewall.
- Keep Windows updated.
- Be careful with flash drives, see this post on
USB/flash drive safety.
- Don't click on links or open attachments in e-mail that you weren't expecting, and read your e-mail in text only mode.
- Stay away from P2P software, even with a clean P2P program, their networks are often riddled with malware.
- Keep your other applications updated, there are vulnerabilities that rely on exploits through other programs like Java, Microsoft Office, Adobe Reader, Flash, and others.
- Run a program like
Secunia Software Inspector Scan to see what programs do need to be updated.
- To make your browsing more secure, use Firefox with the NoScript and Adblock Plus add-ons.
- And most of all, if something looks questionable, stay away from it.